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Acta Polytechnica CTU Proceedings 2(1): 3–20, 2015

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doi: 10.14311/APP.2015.02.0003

The Golden Age of Cataclysmic Variables and Related Objects
(Old and News)

F. Giovannelli1, L. Sabau-Graziati2

1INAF - Istituto di Astrofisica e Planetologia Spaziali, Area di Ricerca di Tor Vergata, Via Fosso del Cavaliere, 100 -
I00177 Roma, Italy

2INTA- Dpt. Cargas Utiles y Ciencias del Espacio, C/ra de Ajalvir, Km 4 - E28850 Torrejón de Ardoz, Madrid, Spain

Corresponding author: franco.giovannelli@iaps.inaf.it

Abstract

In this paper we review cataclysmic variables (CVs) discussing several hot points about the renewing interest of today

astrophysics about these sources. We will briefly discuss also about classical and recurrent novae, as well as the intriguing

problem of progenitors of the Type Ia supernovae. This paper is an extended and updated version of the review by

Giovannelli (2008). Because of limited length of the paper and our knowledge, this review does not pretend to be

complete. However, we would like to demonstrate that the improvement on knowledge of the physics of our Universe is

strictly related also with the multifrequency behaviour of CVs, which apparently in the recent past lost to have a leading

position in modern astrophysics.

Keywords: cataclysmic variables - dwarf novae - intermediate polars - polars - novae - optical - spectroscopy - photometry

- sub-mm - IR - radio - UV - X-rays - multifrequency astrophysics - X-ray binary systems - high energy astrophysics.

1 Introduction

In the 1950s it was recognized that the various phe-
nomena displayed by the CVs are all the consequence
of accretion of matter onto a white dwarf (WD) from a
low mass donor star (e.g., Warner, 1976; 1995a). CVs
are binary systems in which the primary component is
a WD (Mwd ∼ 1 M�) and the secondary is a late type
Main Sequence star (Ms ≤ 1 M�) (e.g., Smak, 1985a).

Mass transfer is strongly depending, besides the or-
bital parameter of the system, on the magnetic field
intensity at the surface of the primary. Such process
produces a large fan of behaviour that are detectable in
different energy ranges: from radio to X-rays, and even
in γ-rays. The orbital periods of CVs are ranging from
∼ 80 m to ∼ 12 h with a distribution showing a gap
between 2 and 3 hours, in which few systems have been
detected. In the past this gap was empty and this was
the reason because was nicknamed ’period gap’.

More than 600 CVs are known, most of them discov-
ered through optical observations, and some, especially
those in which the magnetic field of the WD is strong,
discovered through X-ray observations, but with the de-
tectors of the second and further generations, since CVs
are in general not very bright in X-ray energy range.
Those with known or suspected orbital period are listed
by Ritter & Kolb (1998).

The first CV detected in the X-ray range, with

rocket experiments, was the dwarf nova SS Cyg (Rap-
paport et al., 1974; Heise et al., 1978). The UHURU
satellite detected two CVs, which were not recognized
as such. Warner (1976) proposed the identification of
4U 1249-28 with EX Hya, and the variable AM Her,
which on further optical studies was recognized as a
CV (Forman et al., 1978). The magnetic field in these
two systems is strong (≈ 107–108 G). A few dozen CVs
were detected in X-rays with HEAO-1 satellite, with
EXOSAT, and with the Einstein satellite (e.g., reviews
of Cordova & Mason, 1983; Cordova, 1995). Later Ver-
bunt et al. (1997) recognized 91 CVs from a sample of
162 systems with known or suspected binary periods by
using data of the ROSAT XRT-PSPC All Sky Survey.

Historically, because CVs were observed photomet-
rically and without seeming to follow any regular pat-
tern, they were named with the term cataclysmic (from
the Greek word kataklysmos = flood, storm; Hack &
la Dous, 1993). As collecting of observational data
progressed it became apparent that these objects were
regular binary systems which for some reason changed
in brightness; some of them also regularly (Recurrent
Novae and Dwarf Novae) while some others only once
(Classical Novae). Therefore the classification of CVs
was based on the optical outburst properties, by which
one may distinguish four groups of CVs: (i) classical
novae; (ii) recurrent novae; (iii) dwarf novae; (iv) nova-

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F. Giovannelli, L. Sabau-Graziati

like objects (e.g., Giovannelli & Martinez-Pais, 1991
and references therein; Ritter, 1992; Giovannelli, 2008).
This classification, however, is neither self-consistent
nor adequate and it is much better to consider primarily
the observed accretion behaviour (Smak 1985b). One
obvious advantage of such an approach is connected
with the time scales of various accretion phenomena,
which are sufficiently short to avoid any major obser-
vational bias: the mass accretion rates in CVs usually
range from 10−11 to 10−8 M� yr

−1 (Patterson, 1984);
the time scales are from tens of seconds (oscillations in
dwarf novae at outbursts) to years (super-outbursts of
SU UMa stars or long term variations in VY Scl stars).

However, in the class of nova-like objects there are
two sub-classes: the DQ Her stars and the AM Her
stars. In these sub-classes of CVs the WDs possess mag-
netic fields with intensity enough high for dominating
the accretion disk and all the phenomena related to it.
These classes of magnetic CVs, whose names are coming
from the prototypes DQ Her and AM Her took later the
names of Intermediate Polars and Polars, respectively.
A short history of their discovery has been discussed by
Warner (1995b). Fundamental papers about these sub-
classes are those by Patterson (1994), Warner (1996).
The class of IPs has been split into two subclasses with
relatively large and relatively weak magnetic field (Nor-
ton et al., 1999). One example of a system belonging
to the latter subclass is DO Dra (previously registered
as YY Dra) (Andronov et al., 2008).

There is another class of CVs, the rare AM Canum
Venaticorum (AM CVn) star systems. They have ex-
tremely short orbital periods between ∼ 10 − 65 min-
utes. Their spectra do not show evidence for hydro-
gen. They appear to be helium–rich versions of CVs
(e.g. Warner, 1995c; Nelemans, 2005). There is an old
suggestion, that in these systems the mass transfer is
driven by gravitational wave radiation losses, proposed
by Paczyński (1967), after the discovery of the proto-
type with an orbital period of ∼ 17 minutes (Smak,
1967).

Depending on the magnetic field intensity at the
WD, the accretion of matter from the secondary star
onto the primary can occur either via an accretion disc
(in the so-called Non-Magnetic CVs: NMCVs) or a
channelling through the magnetic poles (in the case of
Polars: PCVs) or in an intermediate way (in the case
of Intermediate Polars: IPCVs).

CVs in a time scale of order between weeks and years
flare up almost periodically, about few magnitudes in
optical wavelengths; the duration of the outbursts is
much shorter than the recurrence time. Typical light
curves for classical novae and dwarf novae of the U Gem,
Z Cam, and SU UMa types can be seen in Ritter (1992)
and e.g. in Giovannelli (2008).

The recurrence time-scale of outbursts in dwarf no-

vae is correlated with their amplitude and the outburst
duration is depending on the orbital period (Warner,
1987).

In PCVs the WD magnetic field is strong enough to
make the Alfvén radius greater than the circularization
radius, so no accretion disc is formed and the accretion
structure is fully governed by the magnetic field, which
canalize the accreting matter across the field lines. Ow-
ing to the intense magnetic field (∼ 10–200 MG), the
WD rotation is synchronized with the binary orbital
period (a few hours). However, there are few systems
(V1432 Aql, BY Cam, V1500 Cyg, V4633 Sgr, and CD
Ind) in which Pspin and Porb differ by around 2% or
less. These are assumed to be polars that have been
disturbed from synchronism by a recent nova explosion
(Norton, Sommerscales & Wynn, 2004, and the refer-
ences therein).

IPCV WDs have moderate magnetic fields (order
of a few MG); the Alfvén radius is smaller than the
circularization radius but it is greater than the WD ra-
dius. Therefore an accretion disc is formed in these
systems but being disrupted at its inner region. In
IPCVs matter follows again the magnetic field lines
but just inside the Alfvén radius. The rotating WD is
asynchronous with the binary orbital period (Pspin �
Porb). However, there are few systems that may be
best described as nearly synchronous intermediate po-
lars (V381 Vel, RXJ0524+42, HS0922+1333, and V697
Sco) (Norton, Sommerscales & Wynn, 2004, and the
references therein). Two of these systems lie in the
‘period gap‘. Probably all four systems are IPs in the
process of attaining synchronism and evolving into po-
lars.

The last group defined by the accretion structure
criterion, NMCVs, includes those systems whose WD
magnetic fields are not relevant in governing the accre-
tion structure. In these systems the accretion disc ex-
tends down to the WD surface and a boundary layer is
formed. This family shows a great diversity of observa-
tional behaviour; for this reason the historical criterion
of classification is, in this case, more appropriate for
distinguishing their sub-classes. However, it is simply
an attempt of classification for lack of a more general
physical classification (e.g., Giovannelli, 1991 and refer-
ences therein). Indeed, in general, we can consider the
WD of a CV as a gravimagnetic rotator, characterized
by a mass M — accreting matter at a rate Ṁ from the
optical companion (the secondary star) — rotating with
a velocity ~ω and having a magnetic moment ~µ, not nec-
essarily coaxial with the rotational axis (Lipunov, 1987;
1991). Then the accreting system is completely charac-
terized by the following physical parameters: mass M,
accretion rate Ṁ, rotational velocity ~ω, and magnetic
moment ~µ. In the plane spin period of the WD (and in
general of the compact object) – gravimagnetic param-

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The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

eter y = Ṁ/µ2 it is possible to find any sort of physical
conditions of gravimagnetic rotators, as discussed by
e.g. Giovannelli (1991).

Figure 1 shows the positions of few CVs in the dia-
gram spin period of the WD – gravimagnetic parameter
(in unit -42): AM Her (Terada et al., 2010 and refer-
ences therein), AE Aqr (Patterson, 1979; de Jager et al.,
1994; Wynn, King & Horne, 1997), DQ Her (Patterson,
1994; Zhang et al., 1995), EI UMa (Reimer et al., 2008),
SS Cyg (Giovannelli & Sabau-Graziati 2012a). It ap-
pears evident the power of this diagram obtained by
Lipunov (1987) using 1 M� white dwarf. The polars
AM Her and AE Aqr lie in the zone of propeller, how
they must stay, while the IPCV DQ Her (the prototype
of this class), EI UMa (a very well known IPCV), and
SS Cyg (whose nature as IPCV is claimed by Giovan-
nelli’s group on the base of many circumstantial proofs
and also because of a cogent similarity with EI UMa,
e.g. Giovannelli & Sabau-Graziati 2012a,c) lie just in
the zone predicted by Lipunov for such objects.

Figure 1: The positions of several CVs in Lipunov’s di-
agram calculated for 1 M� white dwarf (after Lipunov,
1987).

As recalled by Giovannelli & Sabau-Graziati (1999),
it is evident that the properties of an outburst in CVs
depend crucially on the accretion rate, the mass of the
WD, and the chemical composition of its hydrogen rich
envelope in which the thermonuclear runaway occurs.
And the accretion process onto the WD is strongly in-
fluenced by its magnetic field intensity. Indeed, the
three kind of CVs (non-magnetic, polars, and interme-
diate polars) obey to relationships between the orbital
period of the system and the spin period of the WD
(Warner & Wickramasinghe, 1991), where the magnetic
field intensity plays a fundamental role. The orbital
evolution of CVs, and hence the mass-transfer rate (Ṁ)
from the secondary to the white dwarf is driven by mag-

netic braking of the secondary for long-period systems
(Porb > 3 hr) and gravitational radiation for short-
period systems (Porb < 2 hr).

However, such a gap — which was believed true for
long time — is now partially filled by the SW Sex sys-
tems (e.g. Rodriguez-Gil, 2003; Rodriguez-Gil et al.,
2007). The apparent ’period gap’ was due to a smaller
number of systems having orbital periods in such an
interval, which were escaping from the observations.

Therefore the investigation on the magnetic field in-
tensities in WDs is crucial in understanding the evolu-
tion of CVs systems. The fundamental parameters to
be searched are the magnetic moment, the mass accre-
tion rate and the orbital parameters of the systems. In
this way it will be possible to fulfill the plane log Pspin–
log Porb, where a priori there are not restricted ranges of
magnetic moment | ~µ |, or special correlations between
Pspin and Porb and | ~µ |. The distribution of objects
in that diagram is owed to the interaction of braking
torques and accretion torques, with the superposition
of the observed or implied variations of the accretion
rate on long time scale (> 102 yr), acting on a contin-
uum of magnetic moments. In this way each system is
completely described by those physical parameters.

Davis et al. (2008) applied population synthe-
sis techniques to calculate the present day number of
two types of WD-main sequence star (WDMS) binaries
within the ’period gap’. The first are post-common en-
velope binaries with secondary stars that have masses
0.17 ≤ Ms/M� ≤ 0.36 (gPCEBs), such that they will
commence mass transfer within the period gap. The
second type are systems that were CVs at some point
in their past, but detached once they evolved down in
orbital period to ≈ 3 h as a consequence of disrupted
magnetic braking, and are crossing the ’period gap’ via
gravitational radiation (dCVs). They predicted an ex-
cess of dCVs over gPCEBs within the ’period gap’ of
∼ 4 to ∼ 13. This excess is revealed as a prominent peak
at the location of the ’period gap’ in the orbital period
distribution of the combined gPCEB and dCV popula-
tion. They suggest that if such a feature is observed
in the orbital period distribution of an observed sam-
ple of short orbital period WDMS binaries, this would
strongly corroborate the disruption of magnetic brak-
ing.

Willems et al. (2005) and Willems et al. (2007) by
using population synthesis tools studied the population
of NMCVs with orbital periods 1) < 2.75 h, and 2)
> 2.75 h, respectively.

1) A grid of detailed binary evolutionary sequences
was calculated and included in the simulations to take
account of additional angular momentum losses beyond
that associated with gravitational radiation and mass
loss, due to nova outbursts, from the system. As a
specific example, Willems et al. (2005) considered the

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F. Giovannelli, L. Sabau-Graziati

effect of a circumbinary disc to gain insight into the in-
gredients necessary to reproduce the observed orbital
period distribution. The resulting distributions showed
that the period minimum lies at about 80 minutes, with
the number of systems monotonically increasing with
increasing orbital period to a maximum near 90 min-
utes. There is no evidence for an accumulation of sys-
tems at the period minimum, which is a common fea-
ture of simulations in which only gravitational radiation
losses are considered. The shift of the peak to about 90
minutes is a direct result of the inclusion of systems
formed within the period gap.

2) The population of NMCVs with unevolved main–
sequence–like donors at orbital periods greater than
2.75 h was investigated. In addition to the angular
momentum losses associated with gravitational radia-
tion, magnetic braking, and mass loss from the sys-
tem, Willems et al. (2007) also included the effects
of circumbinary discs on the evolution. For a fractional
mass input rate into the disc, corresponding to 3×10−4
of the mass transfer rate, the model systems exhibit a
bounce at orbital periods greater than 2.75 hr. The
simulations revealed that: i) some systems can exist
as dwarf novae throughout their lifetime, ii) dwarf no-
vae can evolve into novalike systems, and iii) novalike
systems can evolve back into dwarf novae during their
postbounce evolution to longer orbital periods. Among
these subclasses, novalike cataclysmic variables would
be the best candidates to search for circumbinary discs
at wavelengths ≥ 10 µm. The theoretical orbital period
distribution is in reasonable accord with the combined
population of dwarf novae and novalike systems above
the period gap, suggesting the possibility that systems
with unevolved donors need not detach and evolve be-
low the period gap as in the disrupted magnetic braking
model. Experimental data are necessary for checking
the validity of theoretical predictions.

The field strength distribution of MCVs differs from
that of single WDs, although both cluster around 30
MG. The distribution of isolated WDs extend on a wide
range of magnetic field strength (∼ 105–109 G), whilst
in accreting WDs of CVs, as far as is presently known,
there is a lack of systems at both high and low field
strengths. However, the apparent absence of low field
MCVs might be explained by the IPs, which generally
have unknown field strengths, and the lack of high field
systems is still not understood (e.g., Beuermann, 1998).
Wynn (2000) discussed the problem of accretion flows
in MCVs. On the base of the ratio Pspin/Porb he di-
vided the MCVs in three classes: class 1, class 2 and
class 3 if such a ratio is � 0.1, ∼ 0.1, and � 0.1,
respectively. For the systems in class 1 the disc equi-
librium condition is clearly satisfied. Those in class 2
are very unlikely to possess accretion discs. The sys-
tems in class 3 are EX Hya-like systems which lie below

the ’period gap’ and cannot possibly contain accretion
discs. These are EX Hya, HT Cam, RXJ1039.7-0507,
and V1025 Cen. These all have Pspin/Porb > 0.1 and
Porb < 2 hr. DD Cir and V795 Her lie within the ’pe-
riod gap’ with Pspin/Porb ∼ 0.1 and may be included
in the class 2 (Norton, Somerscales & Wynn, 2004, and
the references therein). Wynn (2000) crudely classified
the MCVs according to the magnetic moment and or-
bital period. EX Hya systems have magnetic moment
similar to IPs above the ’period gap’ and comparable to
the weakest field AM Her-like systems. This indicates
that MCVs above the ’period gap’ will evolve to long
spin periods below it. Norton, Wynn & Somerscales
(2004) investigate the rotational equilibria of MCVs.
They predict that IPCVs with µ ≥ 5×1033 G cm3 and
Porb > 3 hr will evolve into PCVs, whilst those with
µ ≤ 5 × 1033 G cm3 and Porb > 3 hr will either evolve
into low field strength polars that are presumably un-
observable, and possibly EUV emitters, or into PCVs
when their fields, buried by high accretion rate, revive
when the mass accretion rate reduces.

Warner (1996) deeply discussed torques and insta-
bilities in IPs on the base of measured spin periods of
the primaries and found several important relationships
between fundamental parameters of these systems, such
as log Ṁ vs log Porb, log µ33 vs log Ṁ17, log LX vs log Ṁ,
as shown in Fig. 2 in the left, central, and right panels,
respectively. There is a range of magnetic moments µ
and mass transfer rates in which synchronized rotation
of the primary can occur even though it possesses an
accretion disc.

Ak et al. (2010), using available astrometric and
radial velocity data, computed the space velocities of
CVs with respect to the Sun and investigated kinemat-
ical properties of various sub-groups of CVs. The or-
bital period distribution of CVs in the refined sample
of 159 systems resembles that of the whole sample of
CVs (e.g. Connon Smith, 2007). Ak et al. (2010)
found that the mean kinematical age (MKA) of the
159 systems is MKA159 = 5 ± 1 Gyr. In the sample,
134 of 159 systems are non magnetic (NMCV) having
MKANMCV = 4.0±1.0 Gyr. In the sub–sample of NM-
CVs, 53 of 134 have Porb < 2.62 h and their MKA is
5.0±1.5 Gyr, whilst 81 of 134 systems have Porb > 2.62
h and their MKA is 3.6±1.3 Gyr. This means that CVs
below the ’period gap’ are older than systems above the
gap. This result in agreement with the standard evolu-
tion theory of CVs. The selection of 2.62 h as the border
between the two groups of systems lies roughly in the
middle of the ’period gap’, where systems have been
detected. This means that the ’period gap’ does not
exist anymore and the systems inside this ’gap’ are just
frontier objects between systems experiencing gravita-
tional radiation and those experiencing magnetic brak-
ing. The reason because they are not so numerous as

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The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

Figure 2: Left panel: Intermediate polars in the plane Ṁ-Porb. The two lines are lines of disc stability: stable
above and dwarf nova outburst below. They were computed for f=0.2 and 0.3, with Rout = f × a, being a the
separation of the two stars in the system (by courtesy of Warner, 1996). Central panel: Magnetic moment in
units of 1033 G cm3 versus mass accretion rate in units of 1017 g s−1. Boundaries for white dwarf mass M1 = 1
M� and M1 = 0.6 M� have been computed for Porb = 4 h (by courtesy of Warner, 1996). Right panel: Mass
transfer rate onto white dwarf versus (2-10 keV) X-ray luminosity (by courtesy of Warner, 1996).

those placed at sides could be the relative shorter time
of permanence in the ’gap’, and then difficult to be de-
tected.

Our opinion is that a more appropriate investiga-
tion of the class of the so-called IPCVs is necessary. In-
deed, such systems could show surprises if deeply stud-
ied, as for instance occurred for SS Cyg. This system,
usually considered a NMCV because of a classification
once made by Bath & van Paradijs (1983), whilst since
1984 it has been claimed as an IPCV by Giovannelli
et al. (1985) and later confirmed several times (e.g.,
Giovannelli & Martinez-Pais, 1991; Giovannelli, 1996;
Giovannelli & Sabau-Graziati, 1999, Gaudenzi et al.,
2002). Giovannelli & Sabau-Graziati (2012a) discussed
all the circumstantial proofs in favor of the magnetic
nature of SS Cyg, as well as those adverse, concluding
with reasonable certainty that its nature is magnetic,
being the magnetic field intensity B = 1.7 ± 0.8 MG,
in agreement with the value (B ≤ 1.9 MG) derived by
Fabbiano et al. (1981) by using X-ray, UV and optical
coordinated measurements.

This would teach a lesson: it is mandatory to ob-
serve CVs for long time in order to follow at least a
whole period of the binary system between two succes-
sive outbursts. This is, of course, possible only for sys-
tems like dwarf novae where the almost periodical out-
bursts occur in time scales of weeks-months. Networks
of robotic telescopes can help in this matter (Giovan-
nelli & Sabau-Graziati, 2012b)

However, we can say that CVs form a broad stel-
lar family of highly variable and dynamical members.
When it comes to explaining particulars about, e.g., the
detailed interaction between the transferred matter and
the WD’s atmosphere; irregularities within regular pho-
tometric behaviour; turbulent transport in the disc; or
the final fate of these objects, more is missing than what
is known, rendering their study ever more challenging.

At least, CVs are natural multi–wavelength laborato-
ries offering us the possibility of studying in detail the
behaviour of plasma and radiation under extreme phys-
ical conditions. The understanding of stellar evolution,
electromagnetism and polarization, mass and radiation
transfer or 3-D geometrical effects, in a broad spectral
range from hard X-rays to radio, is mandatory for im-
proving the knowledge of the nature of CVs.

Variability, from milliseconds to hundreds of years,
follows from different physical processes taking place in
these systems and can be studied by means of several
astronomical techniques. As our skills in developing
further these techniques grow our understanding of the
CVs insights also grows; and the more we learn about
CVs the further techniques and theory develop. On the
other hand, it is well known that conclusions obtained
in the field of CVs have been extrapolated, upwards or
downwards in scale, to other fields such as AGNs or
LMXRBs, and vice versa. From such exchanges of in-
formation and results astrophysical research in general
always benefits. Rapid oscillations in CVs are partic-
ularly interesting. As reviewed by Warner (2004), the
rich phenomenology of dwarf nova oscillations (DNOs)
and quasi–periodic oscillations (QPOs) observed in CVs
favour the interpretation that these rapid brightness
modulations (3 to 11,000 s timescales) are magnetic in
nature — magnetically channelled accretion from the
inner accretion disc for DNOs and possible magnetically
excited traveling waves in the disc for QPOs. There is
increasing evidence for the magnetic aspects, which ex-
tend to lower fields the well–known properties of strong
field (PCVs) and intermediate strength field (IPCVs)
CVs. The result is that almost all CVs show the pres-
ence of magnetic fields on their WD primaries, although
for many the intrinsic field may be locally enhanced by
the accretion process itself. There are many behaviour
that parallel the QPOs seen in X-ray binaries, with

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F. Giovannelli, L. Sabau-Graziati

high– and low–frequency X–ray QPOs resembling, re-
spectively, the DNOs and QPOs in CVs. Other papers
about rapid oscillations in CVs are those by Warner &
Woudt (2005) and Pretorius, Warner & Woudt (2006).

The current estimate of the space density of CVs is
of ∼ 3 × 10−6 pc−3 (Warner, 2001). This may be a
significant underestimate of CVs space density, as dis-
cussed by Patterson (1984). Although densities from
the most comprehensive optical Palomar–Green survey
raises the estimate at (3−6)×10−6 pc−3, X-ray All-Sky
surveys give densities of ∼ 1 × 10−5 pc−3 for detected
systems of low Ṁ in hard X-rays (Patterson, 1998).
Then from observational point of view, it is necessary
an intensive search for the faint CVs predicted by pop-
ulation synthesis with orbital periods at ∼ 80 − 100
min that have passed through the orbital period min-
imum at ∼ 78 min and have increasing orbital peri-
ods. This research must be done among the low Ṁ
systems detected by X-ray surveys. Thanks to its high
sensitivity, INTEGRAL is very useful for this purpose.
Up to now, it discovered several new faint CVs, with
Porb > 3 hr, and only one with Porb < 3 hr (e.g. Šimon
et al., 2006; Hudec et al., 2008). High speed photom-
etry of faint CVs have shown that: i) 1 of 10, TV Crv
has Porb = 1.509 hr (Woudt & Warner, 2003); ii) 5
of 13 have Porb < 2 hr (Woudt, Warner & Pretorius,
2004); iii) 1 (CAL 86) of 12 has Porb = 1.587 hr (Woudt,
Warner & Spark, 2005); iv) 3 of 11 have Porb > 3 hr
(Witham et al., 2007).

For reviews about CVs see the fundamental papers
by Robinson (1976), Patterson (1984, 1994), Hack & la
Dous (1993), and the books of Warner (1995a) and Hel-
lier (2001). More recent reviews are those by Connon
Smith (2007), and Giovannelli (2008). The long review
The Impact of Space Experiments on our Knowledge of
the Physics of the Universe by Giovannelli & Sabau-
Graziati (2004) contains also a part devoted to CVs.

2 Multifrequency Emissions

In CVs there are several components that are responsi-
ble for the total emission. Deep discussions about these
components can be found in the literature, but there
is a recent review by Giovannelli (2008, and the refer-
ences therein) that exhaustively summarize their con-
tributions to the total multifrequency emission of CVs.
Briefly, such components are:

a) The secondary stars are cool main sequence
stars with spectral type ranging from G8 to M6, corre-
sponding to temperatures from 5,000 to 3,000 K. Their
contribution is mainly in red and IR regions of the elec-
tromagnetic spectrum.

b) The primary stars. The temperatures of WDs
are known only in few cases: when they belong to high
inclination systems, or when they accrete matter with a

very low mass transfer rate. However, the WD temper-
atures range between 10,000 and 50,000 K (Sion, 1986;
1991). Urban & Sion (2006) found that the WDs in
CVs above the period gap are hotter and more accre-
tion heated (Teff = 25, 793 K) than those below the gap
(Teff = 18, 368 K).

Therefore WDs are expected to radiate essentially
in the UV, but they can be visible also in the optical
range if they are not too hot.

c) The accretion disc: it does not have a homo-
geneous temperature, but spans a large range. Since
the temperature distribution in discs is poorly known,
in order to obtain a rough evaluation of their contri-
bution to the total emission it is necessary to evaluate
the contributions at different frequencies of a synthetic
disc constituted of black bodies at different tempera-
tures, the temperature distribution being that of a sta-
tionary accretion disc (e.g., la Dous, 1994). It then
appears evident that the contribution of such an accre-
tion disc is important in the whole range between EUV
and IR, depending on the choice of the disc parame-
ters. Furthermore, the UV radiation can be supplied
from a zone in the vicinity of the WD (some ten stellar
radii), which could contain any optically thick material
left there. However, the argument of accretion discs de-
serves an important comment. It appeared evident that
the viscosity of matter inside the accretion discs plays
a fundamental role in the description of physical pro-
cesses occurring there. In spite of numerous attempts in
determining such a viscosity, the physical nature of that
still remains largely indeterminate. The best training
for study the viscosity is the subclass of dwarf novae,
showing quasi-periodic outbursts which occur on a time
scale from weeks to months (or even years) and are due
to non-stationary accretion.

Meyer & Meyer-Hofmeister (1981, 1982, 1983)
firstly discussed the physical mechanism responsible for
dwarf nova outbursts which is connected with the ther-
mal instability of the disc which occurs in the tempera-
ture range corresponding to the ionization of hydrogen.
Soon after Smak (1984a,b) extended the study of such
a mechanism. The details are summarized by Smak
(2002).

It is important to point out that the shapes of dwarf
nova light curves, which depend on a number of rele-
vant parameters, depend also on viscosity. In particu-
lar, the characteristic time-scales observed during out-
bursts depend on the viscous time-scale. This provides
an important and almost unique opportunity of obtain-
ing some constraints on viscosity or – within the α disc
approach (Shakura & Sunyaev, 1973) – of an empirical
determination of α.

For a more complete and detailed discussion of dwarf
novae and models of their outbursts – see reviews by
Cannizzo (1993), Osaki (1996), and Lasota (2001).

8



The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

d) The boundary layer: a very important zone
for the emission is that of the transition between the ac-
cretion disc and the WD surface, namely the boundary
layer. It is possible that all, or at least a significant frac-
tion, of the kinetic energy of the material contained in
the accretion disc must be radiated away within the ge-
ometrically very small boundary layer in order to have
the possibility of the material accreting onto the WD’s
surface. Then, whatever the situation, one can assume
the presence of a strong X-ray source at the boundary
layer, which will be visible also in the EUV and short-
wavelength UV according to the choice of disposable
theoretical parameters.

Most of the radiation then comes from the accre-
tion disc and boundary layer, which contribute roughly
50% each. From the accretion disc the radiation is es-
sentially emitted in the optical and UV, whilst from
the boundary layer — optically thick (which occurs at
high accretion rates) — the radiation is emitted in the
soft X-ray range; when the accretion is at low rates the
boundary layer is optically thin and appears as a hard
thermal bremsstrahlung source. These predictions have
been tested experimentally, comparing the observations
of CVs in optical, UV and soft X-ray ranges (e.g. Wood
et al., 1989; Horne et al., 1994).

e) The gas stream: it is definitively optically thin
and cool and contains rather little material; so, proba-
bly, its contribution to the total emission of CVs is neg-
ligible at all frequencies as source of continuum, whilst
it could contribute to the formation of lines in the red
and IR regions.

f ) The hot line: the energy excess zone in the
place where the stream comes to the disc is a shock
wave. This zone was previously known as ‘the hot spot‘.
Its structure and radiation characteristics are still an
open problem; it is visible in many systems in optical
photometry (less in the IR and never in the UV) as a
periodically recurring hump in the orbital light curve.
Its temperature must be ≤ 10, 000 K.

g) Hot corona or chromosphere: is a shell of
optically thin and rather hot gas, below and above the
accretion disc. X-ray and UV line radiation are ten-
tatively attributed to it, whilst it probably does not
contribute to the UV, optical or IR continuum emis-
sion.

Usually no radio emission from CVs has been mea-
sured. Only upper limits for individual systems of order
of a few mJy are available (e.g ≤ 10 mJy in SS Cyg —
Cordova, Mason & Hjellming 1983). However, recently,
Körding et al. (2008) detected a radio flare from SS
Cyg peaked at 1.1 mJy with a duration of order 20
days, above the upper limit of 0.08 mJy. This radio
flare was simultaneous with the optical long outburst
peaked at about 1 Jy.

Pringle & Wade (1985) computed the contribution
functions of the most important components of a cata-
clysmic system, previously discussed. The plot can be
found also in Fig. 6 of Giovannelli’s paper (2008).

During quiescence dwarf novae emit essentially hard
X-rays (∼ 0.1–4.5 keV) and the flux distribution is
rather well approximated by a thermal bremsstrahlung
with KTbrems ≈ 10 keV (Cordova & Mason, 1983). A
direct correlation between the hard X-ray/optical fluxes
ratio and Hβ equivalent width has been found by Pat-
terson & Raymond (1985).

During outburst dwarf novae emit soft X-rays (0.18–
0.5 keV) with an increase of the flux of the order of 100
or more, although most of the radiation is hidden in the
EUV range (Cordova & Mason, 1984). The soft X-ray
spectra can be fitted either with black bodies at KTbb ≈
25–30 eV or, alternatively, with bremsstrahlung spectra
at KTbrems ≈ 30–40 eV.

The most important features are the anti-correlation
between the hard and soft X-ray emissions during the
outburst cycle and the correlation between soft X-ray
and optical emissions, as measured for SS Cyg (Watson,
King & Heise, 1985), or — what is the same — anti–
correlation between the hard X-ray and optical emission
(Ricketts, King & Raine, 1979). During an outburst of
SS Cyg there is also the correlation of optical and EUV
emissions, that are anti–correlated with the hard X-ray
emission detected by RXTE (Wheatley et al., 2003).

What does that mean? The UV flux and the bulk of
optical flux in dwarf novae and nova-like stars originate
in the accretion disc. The IR flux observed during qui-
escence and possibly some of the optical flux come from
the secondary late-type star. The rise to an outburst ei-
ther occurs simultaneously at all wavelengths when it is
slow, or progressively starts later with decreasing wave-
lengths when it is fast, since ever more central hotter
parts of the disc become involved. Indeed, several dwarf
novae have been observed in the UV and optical during
the rise to maximum outburst brightness and their be-
haviour are quite similar: the UV rise lags the optical
rise by up to a day (e.g., VW Hyi: Hassall et al. (1983);
CN Ori and RX And: Cordova, Ladd & Mason (1986);
WX Hyi: Hassall, Pringle & Verbunt (1985). With re-
spect to the optical band, this lag is similar also in the
EUV region covered by the Voyager (50–1200 Å) for SS
Cyg (Polidan & Holberg, 1984) and VW Hyi (Polidan
& Holberg, 1987).

This fact strongly supports the origin of the out-
burst being in the cooler outer part of the disc rather
than in the hotter parts near the WD; therefore mini-
nova models for the outbursts are probably excluded
(Cordova & Howarth, 1987). The two models for trig-
gering the outbursts, compatible with the lag observa-
tions, are then:

9



F. Giovannelli, L. Sabau-Graziati

• an instability in the secondary star which allows
the transfer of more mass to the disc;

• a thermal instability in the outer disc, which
results in material stored there being suddenly
transported through the disc. During the decline,
the whole disc cools simultaneously. The contri-
bution to the total emission from the boundary
layer between the disc and the WD surface is in
UV and X-ray ranges: the boundary layer is opti-
cally thin during quiescence and then emits hard
X-rays, but it is optically thick during outburst
and then emits soft X-rays since the radiation is
thermalized before escape (la Dous, 1993).

From the EUVE (Extreme Ultraviolet Explorer) Craig
et al. (1997) have shown the EUV spectra of three
NPCVs in outburst, namely VW Hyi, U Gem and SS
Cyg. VW Hyi shows the softest EUV spectrum peaked
at ∼ 250 Å. However, its boundary layer/disc luminos-
ity ratio — Lbl/Ldisc ∼ 0.2 (Mauche et al., 1991) — is in
contrast with the boundary layer models. In U Gem the
total size of the EUV emitting region is comparable to
that of the WD itself, which indicates that the outburst
in mainly confined to the inner disc/boundary layer re-
gion (Long et al., 1996). Its Lbl/Ldisc ∼ 1. SS Cyg is the
hardest of the three EUV NMCVs. Its EUV spectrum is
rather complex and changed by a factor 100 during the
outburst with an almost constant spectral energy distri-
bution. Quasi-coherent oscillations (∼ 7−9 s) have been
detected in the EUV emission (Mauche, 1996). SS Cyg
shows no EUV emission longward ∼ 130 Å. Mauche,
Raymond & Mattei (1995) found that for SS Cyg the re-
lation Lbl/Ldisc ∼ 1, valid for the boundary layer mod-
els is strongly violated, being this ratio Lbl/Ldisc ∼ 0.07.
And this is one more proof that SS Cyg is not a NMCV
— as already remarked — as claimed by Giovannelli’s
group.

IUE satellite deserves special comments since it was
fundamental in improving the knowledge of CVs. A
detailed review can be found in Giovannelli (2008).
Briefly, the IUE gave significant contributions on:

i) Knowledge of disc accreting and magnetic
CVs, as extensively discussed by Cordova (1995), and
references therein.

ii) Nature of the high velocity winds. Dur-
ing outburst of SS Cyg the spectral emission features
disappear or go into absorption, some of them showing
P Cygni profile (e.g. CIV), which clearly indicate the
presence of high velocity wind from the system. The
emission features appear again when the system is go-
ing into quiescence (Giovannelli et al., 1990).

iii) Boundary layer emission.
Multifrequency observations show that X-ray luminos-
ity at all outburst phases is much lower (about at least
a factor 10) than the UV/optical luminosity from the

disc, as expected from the models (e.g., Mauche, 1998).
This simply means that the boundary layer models are
not correct.

iv) Underlying WD and its photosphere. IUE
provided the first evidence that the WD is heated by
the dwarf nova outburst and subsequently cooled. A
list of nine such systems has been reported by Szkody
(1998). These measurements are very difficult because
of the long quiescence–outburst–quiescence cycles (from
weeks to years). The short outburst period dwarf nova
VW Hyi cooled to 18,000 K (from 20,500 K) in the 14
days before the next outburst began (Verbunt et al.,
1987).

v) Magnetic field of the WD. Indirect evalua-
tions of magnetic field intensities in CV WDs have been
obtained through multifrequency observations (e.g.,
Fabbiano et al., 1981). Our feeling is that the problem
of magnetic fields in WDs has been underestimated in
the studies of CVs.

Too many simplified models of disc accreting and
magnetic CVs have been developed under the hypoth-
esis that CVs can be sharply divided into three classes:
Polar, IP, Non-Magnetic. Magnetic fields are smoothly
varying in their intensities from one class to another.
The discovery in some IPs of a circularly polarized op-
tical emission suggests that these intermediate polars
will evolve into polar systems (e.g., Mouchet, Bonnet-
Bidaud & de Martino et al., 1998). Some evidence of
the continuity between the IPCVs and PCVs is com-
ing from the detection of the SW Sex systems. They
have orbital periods just inside the so-called ’period
gap’, which separates the two classes of IPCVs and
PCVs (e.g. Rodriguez-Gil, 2003 and references therein;
Rodriguez-Gil et al., 2007).

Looking at the homogeneous set of data coming
from the IUE for PCVs and IPCVs, it has been possible
to obtain important information on common properties
and peculiarities of these binaries (de Martino, 1999
and references therein), which render the two classes
rather similar in some of their UV behaviour. Mouchet,
Bonnet-Bidaud & de Martino (1998), and de Martino
(1998) made the hypothesis that the two classes are
evolutionary related.

The far UV vs near UV colour–colour diagram for
MCVs was constructed by de Martino (1999). Such a
diagram was constructed measuring broad band con-
tinua in the IUE short wavelength range (1420–1520 Å
and 1730–1830 Å) and in the long wavelength range
(2500–2600 Å and 2850–2900 Å). Clearly the UV con-
tinua cannot be simply described by a single component
but possess different contributions, as discussed by de
Martino (1999) and already noted in the past, since
1984, by Giovannelli et al. (1985).

Araujo-Betancor et al. (2005) obtained Hubble
Space Telescope (HST) STIS data for a total of 11 PCVs

10



The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

as part of a program aimed at compiling a homogeneous
database of high–quality FUV spectra for a large num-
ber of CVs. Comparing the WD temperatures of PCVs
with those of NMCVs, they find that at any given or-
bital period the WDs in PCVs are colder than those in
NMCVs. The temperatures of WDs in PCVs below the
period gap are consistent with gravitational radiation
as the only active angular momentum loss mechanism.
The differences in WD effective temperatures between
PCVs and NMCVs are significantly larger above the
period gap, suggesting that magnetic braking in PCVs
might be reduced by the strong field of the primary.
Araujo-Betancor et al. (2005) derive a lower limit on
the space density of PCVs of 1.3 × 10−6 pc−3.

3 Renewed Interest for Cataclysmic
Variables

Before the advent of ROSAT X-ray satellite, MCVs
were relegated to a subsection of conferences about
CVs that were mainly concentrated on NMCVs. The
ROSAT satellite discovered many MCVs that even
menaced to overthrow our understanding of the secu-
lar evolution of ‘normal‘ CVs by appearing – appar-
ently inexplicable – in the so–called ‘period gap‘ in the
orbital–distribution of CVs (e.g. Vrielmann & Cropper,
2004). But in spite of this, CVs were not considered,
in general, for many years as principal targets of high
energy X-ray experiments.

At the beginning of the nineties of the last century,
acceleration of particles by the rotating magnetic field
of the WD in intermediate polars in the propeller regime
— AE Aqr -– detected by ground-based Cherenkov tele-
scopes in the TeV passband (e.g. Meintjes et al. 1992),
and TeV emission from the polar AM Her detected by
ground-based Cherenkov telescopes (Bhat et al. 1991)
— measurements never confirmed — were the main rea-
sons of renewed interest for CVs in the high energy as-
trophysicists community.

The INTEGRAL observatory, until the beginning
of 2007, had observed over 70 percent of the sky, with
a total exposure time of 40 million seconds. Bird et
al. (2007) published the third INTEGRAL catalogue of
gamma-ray sources. It contains a total of 421 gamma-
ray objects. Most have been identified as either binary
stars in our Galaxy containing exotic objects such as
black holes and neutron stars, or active galaxies, far
away in space. But a puzzling quarter of sources re-
main unidentified so far. They could be either star sys-
tems enshrouded in dust and gas, or CVs. Integral ob-
serves in the gamma-ray band so it can see through
the intervening material. It has demonstrated that
it can discover sources obscured at other wavelengths.
One surprise has been the efficiency with which Inte-
gral has detected just one minor subclass CVs, the so-

called IPCVs. Initially astronomers were not sure that
CVs would emit gamma rays. Indeed, INTEGRAL has
already shown that only about one percent of them
do. This fact overbearingly renewed the interest for
CVs, apparently fallen into disgrace in favour of binary
systems containing either neutron stars or black holes.
The fourth IBIS/ISGRI catalog reports 331 additional
sources when compared to the third catalog. Of these,
120 are associated with extragalactic sources, while only
25 are associated with known Galactic sources, and the
remainder are so far unidentied (Bird et al. 2010). CVs
constitute ∼ 5% of the total sources.

Moreover, since the CVs measured by the INTE-
GRAL observatory are magnetic in nature, the interest
for such class of objects has been addressed to evolu-
tionary problems.

The long–standing fundamental predictions of evo-
lution theory are finally being tested observationally.
All facets of the accretion process in CVs, including
variability, disc winds and jets, are universal with ac-
creting WDs, neutron stars, and black holes (Knigge,
2010, 2011). Knigge, Baraffe & Patterson (2011) ex-
tensively discussed the reconstruction of the complete
evolutionary path followed by CVs, based on the ob-
served mass–radius relationship of their donor stars, fol-
lowing Knigge (2006) that discussed the observational
and theoretical constraints on the global properties of
secondary stars in CVs using the semi–empirical CV
donor sequence, and concluded that most CVs follow a
unique evolutionary track.

In the standard model of CV evolution, angular–
momentum–loss (AML) below the period gap are as-
sumed to be driven solely by gravitational radiation
(GR), while AMLs above the gap are usually described
by a magnetic bracking (MB) (Rappaport, Verbunt &
Joss (1983). Knigge, Baraffe & Patterson (2011) with
their revised model, found the optimal scale factors fGR
= 2.47 below the gap and fMB = 0.66 above, whilst
the standard model gives fGR = fMB = 1. This revised
model describes the mass–radius data much better than
the standard model.

The sub-class of CVs, named Classical Novae (CNe),
which are the third more powerful stellar explosions in
a galaxy, have been observed as close as a kpc and as
far as galaxies in Fornax cluster. The time to report
on the recent renaissance in studies on CNe thanks to
observations with 8-10m class telescopes, high resolu-
tion spectroscopy, in synergy with observations from
space carried out with Swift, XMM, Chandra, HST,
and Spitzer, coupled with recent advances in the the-
ory of the outburst, seems now in order. Moreover, the
possible connection among some CV-types and SNe-Ia
will definitively justify the renewed interest about CVs.

11



F. Giovannelli, L. Sabau-Graziati

4 Classical and Recurrent Novae

Classical novae are expected to recur on timescales from
100,000 years to just a few decades. The most im-
portant physical parameters controlling this recurrence
timescale are the WD mass, and the mass accretion
rate from the secondary (e.g. Yaron et al. 2005). Once
classical nova (CN) is recorded more than once, it can
be designated as “recurrent” (RN). Since the WD and
the binary system remain intact after an outburst, it is
possible that classical novae may actually be the same
as recurrent novae if observed over a long enough time
period. While the interval between outbursts of recur-
rent novae range from 10 to 100 years, it has been esti-
mated that the time interval for classical novae would
range from about 30,000 years for a 1.3 M� WD to
100,000 years for a 0.6 M� WD. Given long enough - it
is expected that all classical novae will be observed as
recurrent novae.

The long term behaviour of classical old novae, and
the optical behaviour of CNe in outburst were discussed
by Bianchini (1990), and Seitter (1990), respectively.
The books by Cassatella & Viotti (1990) and by Bode
& Evans (2008) are very useful for studying the physics
of classical novae.

Recurrent novae are a rare sub-class of cataclysmic
variable stars; WDs accreting material from a binary
companion in which more than one classical nova-type
outburst has been observed (see the book of Hellier,
2001 for a comprehensive review of CVs). Nova out-
bursts are suspected to be due to a thermonuclear run-
away on the surface of the WD, which releases huge
amounts of thermal energy once a critical pressure is
reached at the base of the shell of accreted material.

One of the most interesting RNe is RS Ophiuchi
(RS Oph). It is an amazingly prolific recurrent nova,
with recorded outbursts in 1898, 1907, 1933, 1945, 1958,
1967, 1985 and 2006 (Schaefer 2010). The short time
between outbursts (∼ 20 yrs) suggests that RS Oph
hosts a massive WD accreting material at a significantly
high rate.

In the latter paper Schaefer discussed not only RS
Oph, but also the photometric histories of all known
galactic RNe.

Classical and recurrent nova outbursts have been re-
cently discussed by Bode (2011a,b) and Evans (2011).
The proceedings of a conference about RS Oph and
recurrent phenomenon can be very useful for details
(Evans et al., 2008). General properties of quiescent
novae have been discussed by Warner (2002). The very
useful book of Bode & Evans (2008) about classical no-
vae examines thermonuclear processes, the evolution of
nova systems, nova atmospheres and winds, the evo-
lution of dust and molecules in novae, nova remnants,
and observations of novae in other galaxies. It includes

observations across the electromagnetic spectrum, from
radio to gamma rays, and discusses some of the most
important outstanding problems in classical nova re-
search.

Of the ∼ 400 known Galactic classical novae, only 10
of them are recurrent. Eight of them harbour evolved
secondary stars, contrary to classical novae that contain
main sequence stars (Darnley et al., 2011). They pro-
pose a new nova classification based on the evolution-
ary state of the secondary star, contrary the current
schemes based on the properties of outbursts. Such
classification contains three groups of novae: i) Main
Sequence Nova (MS–Nova); ii) Sub–Giant Nova (SG–
Nova); and iii) Red Giant branch Nova (RG–Nova).

RNe play an important role in the studies of SN Ia
progenitors (Surina et al., 2011). RNe are likely pro-
genitors of Type–Ia supernovae.

In order to brave this important problem the use of
archival data is the only way to answer the big question.
Now, huge and comprehensive set of archival RN data
go back to 1890.

5 Progenitors of SN Ia

It is well accepted by the community that Type–Ia SNe
are the result of the explosion of a carbon–oxygen WD
that grows to near Chandrasekhar’s limit in a close bi-
nary system (Hoyle & Fowler, 1960). But the debate is
focussed around the different kinds of progenitors. In-
deed, in the past, two families of progenitor models have
been proposed. They differ in the mode of WD mass
increase. The first family is the so–called single degen-
erate (SD) model (Whelan & Iben, 1973), in which the
WD accretes and burns hydrogen–rich material from
the companion. The second family is the so–called dou-
ble degenerate (DD) model, in which the merging of two
WDs in a close binary triggers the explosion (Webbing,
1984; Iben & Tutukov, 1984). The two scenarios pro-
duce different delay times for the birth of the binary
system to explosion. Thus it is hopefully possible to
discover the progenitors of Type–Ia SNe by studying
their delay time distribution (DDT). The DDT can be
determined empirically from the lag between the cosmic
star formation rate and Type–Ia SN birthrate.

The energy released through runaway thermonu-
clear process ejects the majority of the unburnt hydro-
gen from the surface of the star in a shell of material
moving at speeds of up to 1.5 ×103 km s−1. This pro-
duces a bright but short-lived burst of light - the nova.

Although Type–Ia supernova appear to have similar
origin to classical novae, there are key differences. The
most important is that in a classical nova, the ther-
monuclear runaway occurs only on the surface of the
star, allowing the WD and the binary system to remain
intact (e.g. Townsley & Bildsten, 2005). In a Type–Ia

12



The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

supernova, the thermonuclear runaway occurs within
WD itself, completely disrupting the progenitor. This
is reflected in the amount of energy released in the ex-
plosions, with classical novae releasing ∼ 1044 erg, and
Type–Ia supernovae ∼ 1051 erg.

The possible progenitors of SN Ia are: i) Recurrent
Novae; ii) Symbiotic stars; iii) Super-soft sources; iv)
Double WD Binaries; and v) WDs accreting material
from red–giant companions.

i) Recurrent Novae are just a subset of ordinary
novae that happen to go off more than once per century.

As such, they are binary systems with matter flow-
ing off a companion star onto a WD, accumulating on
its surface until the pressure gets high enough to trigger
a thermonuclear runaway that is the nova.

Only 10 RNe are known in our Milky Way galaxy,
including: U Sco (1863, 1907, 1917, 1936, 1945, 1969,
1979, 1987, 1999); T Pyx (1890, 1902, 1920, 1944,
1967); T CrB (1866, 1946); RS Oph (1898, 1907, 1933,
1945, 1958, 1967, 1985, 2006).

To recur with τrec < 100 years, RNe must have:
high WD mass (1.2M� < MWD < MChandra), and high
accretion rate (Ṁ ∼ 10−7 M� yr−1). SN Ia occurs
if: i) the mass ejected for each eruption is less than
the mass accreted onto the WD (Mejected < Ṁ τrec); ii)
the rate of death RNe must be enough to produce the
SN Ia rate (RRNdeath = RSNIa), being RRNdeath = NRN
×(0.2M�Ṁ).

In order to solve the problems we need to know τrec
(recurrence time scale) from archive plates, NRN (num-
ber of RNe in the Milky Way) from archive plates and
AAVSO, Ṁ (mass accretion rate onto WD) from the
average in the last century, Mejected (mass ejected in
eruption) from pre–eruption eclipse timing.

Some results have been obtained for becoming opti-
mists in solving the problem of SN Ia production. In-
deed Schaefer (2011) obtained for CI Aql and U Sco
Mejected << Ṁ τrec).

Thus, WDs are gaining mass and the latter RNe will
collapse as SN Ia. Moreover, for the Milky Way, M31,
and LMC RRNdeath ∼ NRN. Then there are enough
RNe to supply the Type–Ia SN events.

ii) Symbiotic Stars contain WDs efficiently ac-
creting material from the secondary star. In most cases
they steadily burn H–rich material allowing them to
grow in mass. Some of these systems can produce high
mass WDs. In symbiotic RNe (SyRNe) the WD mass
is already very close to Chandrasekhar’s limit. For in-
stance in V 407 Cyg a very massive WD is accreting
material at a rate of ∼ 10−7 M� yr−1 from a Mira–
type companion (Miko lajewska, 2011).

iii) Super–soft Sources are probably WDs that
accrete material and burn hydrogen. Voss & Nielemans
(2008) discovered an object at the position of the Type–
Ia SN2007on in the elliptical galaxy NGC1404 on pre–

supernova archival X–ray images. This result favours
the accretion model (SD) for this supernova, although
the host galaxy is older than the age at which the ex-
plosions are predicted in SD models. However, the DD
model cannot be ruled out by this event because a hot
accretion disc is probably the intermediate configura-
tion of the system, between first WD–WD Roche–lobe
contact and explosion (Yoon, Podsiadlowski & Ross-
wog, 2007).

Greggio, Renzini & Daddi (2008) starting from the
fact that Type–Ia SN events occur over an extended
period of time, following a distribution of delay times
(DDT), discussed theoretical DDT functions that ac-
commodate both ‘prompt‘ and ‘tardy‘ SN events de-
rived by empirically–based DDT functions. Moreover
such theoretical DDT functions can account for all
available observational constraints. The result is that
SD/DD mix of SNIa’s is predicted to vary in a system-
atic fashion as function of cosmic time (redshift).

iv) Double WDs Binaries are systems contain-
ing two WDs that can merge and giving rise to SN
explosion. Yoon, Podsiadlowski & Rosswog (2007) ex-
plored the evolution of the merger of two carbon–oxygen
(CO) WDs. Their results imply that at least some
products of double CO WDs merger may be considered
good candidates for the progenitors of Type–Ia SNe.
Brown et al. (2011) and Kilic et al. (2011) studied a
complete colour–selected sample of double–degenerate
binary systems containing extremely low mass (ELM)
(≤ 0.25 M�) WDs. Milky Way disc ELM WDs have
a merger rate of ≈ 4 × 10−5 yr−1 due to gravitational
wave radiation. The ELM WD systems that undergo
stable mass transfer can account for about 3% of AM
CVn stars. The most important fact is that the ELM
WD systems that may detonate merge at a rate compa-
rable to the estimate rate of underluminous SNe. These
SNe are rare explosions estimated to produce only ∼ 0.2
M� worth of ejecta. At least 25% of ELM WD sam-
ple belong to the old tick disc and halo components of
our Galaxy. Thus, if merging ELM WD systems are
the progenitors of underluminous SNe, transient sur-
veys must find them in both elliptical and spiral galax-
ies.

v) WDs accreting material from red–giant
companions. Observations carried out by Patat et
al. (2008) with VLT–UVES allowed to detect circum-
stellar material in a normal Type–Ia SN. The expansion
velocities, densities and dimensions of the circumstellar
envelope indicate that this material was ejected from
the system prior to the explosion. The relatively low
expansion velocities favour a progenitor system where
a WD accretes material from a companion star, which
is in the red–giant phase at the time of explosion.

Bianco et al. (2011) searched for a signature of a
non–degenerate companion in three years of Supernova

13



F. Giovannelli, L. Sabau-Graziati

Legacy Survey data. They found that a contribution
from WD/red–giant binary system to Type–Ia SN ex-
plosions greater than 10% at 2σ, and than 20% at 3σ
level is ruled out.

Type–Ia SNe are used as primary distance indica-
tors in cosmology (e.g. Phillips, 2005). Phillips (2011)
reviewed the near–infrared (NIR) of Type–Ia SNe con-
cluding that such SNe are essentially perfect standard
candles in the NIR, displaying only a slight dependence
of peak luminosity on decline rate and colour. Lira
(1995) first noted that B–V evolution during the pe-
riod from 30 to 90 days after V maximum is remarkably
similar for all SN Ia events, regardless of light–curve
shape. This fact was used by Phillips et al. (1999) to
calibrate the dependence of the Bmax–Vmax and Vmax–
Imax colours on the light curve parameter ∆m15 (B)
which can, in turn, be used to separately evaluate the
host galaxy extinction. Using these methods for elimi-
nating the effect of the reddening, they reanalyzed the
functional form of the decline rate versus luminosity re-
lationship and gave a value of the Hubble constant of
H0 = 63.3 ± 2.2 ± 3.3 km s−1 Mpc−1.

The use of Type–Ia SNe is also fundamental for
determining some cosmological constraints, such as
ΩM and ΩΛ that fit a ΛCDM models with values of
0.211±0.034 (stat) ±0.069 (sys) using a set of 252 high–
redshift SNe (Guy et al., 2010) and 0.713+0.027−0.029 (stat)
+0.036
−0.039 (sys) using a set of low–redshift nearby–Hubble–
flow SNe (Kowalski et al., 2008), respectively.

In order to explore the difficult topic of the expan-
sion of the Universe it is necessary to know the evolution
of metallicity in old Universe that changes the Hubble
Diagram shape. The proposed space observatory Super
Nova Acceleration Probe (SNAP) is designed to mea-
sure the expansion of the Universe and to determine the
nature of the mysterious Dark Energy that is acceler-
ating this expansion. SNAP is being proposed as part
of the Joint Dark Energy Mission (JDEM) (Stril et al.,
2010), which is a cooperative venture between NASA
and the U.S. Department of Energy. If selected it will
be launched before 2020.

SNAP cannot achieve its main goal without progen-
itor/evolution solution.

For comments and prospects about Type–Ia SN sci-
ence in the decade 2010–2020 see the paper by Howell
et al. (2009).

6 Some Open Questions

Several fundamental questions concerning CVs still re-
main waiting for a proper answer. We will present
briefly only some of them here.

One of them is the lack of a coherent classification,
especially for NLs. On the other hand, in gross features

and in most respects, DN and NLs, as well as quiescent
novae, are almost indistinguishable, although, in addi-
tion to their different outbursts’ behaviour, there ap-
pear to be some further minor differences which are not
yet understood (see Hack & la Dous 1993). The ques-
tion arises of whether the outburst behaviour, the cur-
rent basis of almost all classification is really a suitable
criterion for sorting CVs in physically related groups.
There are also too many exceptions, either systems that
do not fit in any particular group or that can be in-
cluded in several of them, to be able to render the obser-
vational behaviour, at least as it is used at the present,
suitable.

Could CVs be considered simply gravimagnetic ro-
tators? This should be the most suitable approach for
studying them from a physical point of view.

Studies of rotational equilibria of MCVs predict that
IPCVs will evolve either into PCVs or into low field
strength polars — presumably unobservable, and pos-
sibly EUV emitters — depending on their magnetic mo-
ments and orbital periods. Indeed, there are systems,
like EX Hya-type, having magnetic moment similar to
IPCVs above the ’period gap’ and comparable to the
weakest field AM Her-like systems.

Moreover, the detection of several SW Sex systems
having orbital periods inside the so-called ’period gap’
opens a new interesting problem about the continuity
in the evolution of CVs.

The rare AM Canum Venaticorum (AM CVn) stars
have extremely short orbital periods, between 10 and 65
minutes, and their spectra show no evidence for hydro-
gen. They appear to be helium-rich versions of CVs.
They are still waiting for a general model. They are
probably binary systems of two white dwarfs, but even
this is still controversial.

Despite all the work developed during the last
decades, the problem of modeling accretion discs in CVs
is by no means closed, especially in quiescence. Closely
related is the problem of the cause of outbursts. We
really do not know which of the present two families
of models (Disc Instability Models or Secondary Insta-
bility Models) is responsible for the CVs outburst phe-
nomenon, or in which system is each model valid, al-
though Martinez-Pais et al. (1996) gave a contribution
in solving this problem at least in the case of SS Cygni;
they found some evidence for an increase of the mass
transfer rate from the secondary star as the mechanism
responsible for symmetric outbursts. Something simi-
lar can be said about the super-outburst phenomenon
in SU UMa systems.

Gaudenzi et al. (1990), analyzing IUE spectra of
SS Cygni, discussed about the outburst production as
due to the destruction of the accretion disc. The mat-
ter, passing through the boundary layer, slowly accretes
onto the WD. Long and short outbursts correspond to

14



The Golden Age of Cataclysmic Variables and Related Objects (Old and News)

total or partial destruction of the disc, respectively.
Alternatively, could nuclear burning be responsible

of the production of outbursts in CVs? Indeed, nu-
clear burning onto white dwarf’ surface was proposed
by Mitrofanov (1978, 1980) as a mechanism suitable to
generate X-rays in CVs. In spite of this shrewd sugges-
tion, the community of theoreticians did not consider
such a mechanism — certainly possible — worthy of
taking up a part of their time. However, we believe
that this alternative solution in explaining the gener-
ation of outbursts in CVs would deserve theoretician
community’s care. For instance, the white dwarf sur-
face interested in the accretion in the system SS Cygni
has been evaluated as 24% of the total (Gaudenzi et al.,
2002). There, nuclear burning could occur.

Accretional heating by periodic DN events increases
substantially the surface temperature of the WD in CVs
(Godon & Sion, 2002). Then, the envelope thermal
structure resulting from compression and irradiation
should be a crucial component in understanding the
envelope structure of a pre–nova WD.

Another problem still open is connected with the
classification of CVs in three kinds, namely NMCVs,
PCVs and IPCVs. This is, in our opinion, another con-
venient classification, although artificial, probably not
necessary if CVs are studied as gravimagnetic rotators.
In this way a smooth evolution of the systems could be
responsible of the variations of the gravimagnetic pa-
rameters.

Are the IPCVs and PCVs smoothly connected via
the SW Sex-like systems placed just in between? SW
Sex systems have indeed orbital periods belong to the
so-called ’period gap’, and then their presence there
sure cancel that gap.

Could some systems behave in different ways de-
pending on their instantaneous physical conditions?
For this reason they could apparently behave sometimes
as PCVs and sometimes as NPCVs.

An example very clear is that of SS Cygni, usually
classified as a non-magnetic dwarf nova. It has been de-
tected by the INTEGRAL observatory in a region of the
spectrum (up to ∼ 100 keV). This emission is very hard
to be explained without the presence of polar caps in
the WD of the system. Several proofs have been shown
and discussed many times by Giovannelli’s group in or-
der to demonstrate the Intermediate Polar nature of it
(e.g., Giovannelli, 1996, and references therein; Giovan-
nelli & Sabau-Graziati, 1998; 2012a); indeed, SS Cygni
shows characteristics of a NMCV, as well as those of IP
and sometimes even those of polars, although its posi-
tion in the log Pspin–log Porb plane is very close to the
line where IPs lie.

Important results are coming from the SPITZER
space telescope with the detection of an excess (3-8)
µm emission from Magnetic CVs, due to dust (How-

ell et al., 2006; Brinkworth et al., 2007). Gaudenzi et
al. (2011) discussed about the reasons of the variable
reddening in SS Cyg and demonstrated that this red-
dening is formed by two components: the first is inter-
stellar in origin, and the second (intrinsic to the system
itself) is variable and changes during the evolution of a
quiescent phase. Moreover, an orbital modulation also
exists. The physical and chemical parameters of the
system are consistent with the possibility of formation
of fullerenes.

The SPITZER space telescope detected the presence
of fullerenes in a young planetary nebula (Cami et al.,
2010). Fullerenes are the first bricks for the emergence
of the life. Therefore, the possible presence of fullerenes
in CVs opens a new line of investigation, foreboding of
new interesting surprises.

7 Conclusions

At the end of this review it appears evident that the
most suitable approach for studying CVs from a phys-
ical point of view is to consider them as gravimagnetic
rotators.

The detection of several SW Sex systems having or-
bital periods inside the so-called ’period gap’ opens a
new interesting problem about the continuity in the evo-
lution of CVs. Are the IPCVs and PCVs smoothly
connected via the SW Sex-like systems placed just in
between?

In order to fully understand the emission proper-
ties and evolution of CVs, the mass–transfer process
needs to be clearly understood, especially magnetic
mass transfer, as well as the properties of magnetic vis-
cosity in the accretion discs around compact objects.
Consequently, the investigation on the magnetic field
intensities in WDs appears crucial in understanding the
evolution of CVs systems, by which it is possible to gen-
erate classical novae (e.g., Isern et al., 1997) and type-Ia
supernovae (e.g., Isern et al., 1993).

In those catastrophic processes the production of
light and heavy elements, and then the knowledge of
their abundances provides strong direct inputs for cos-
mological models and cosmic ray generation problems.

Acknowledgments This research has made use of
NASA’s Astrophysics Data System.

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	Introduction
	Multifrequency Emissions
	Renewed Interest for Cataclysmic Variables
	Classical and Recurrent Novae
	Progenitors of SN Ia
	Some Open Questions
	Conclusions