Great progress has been achieved in recent years in investigations of the dense degenerate matter and in calculations of white dwarf structure. Nevertheless the classical theory by Chandrasekhar (1967) describes the structure of white dwarf stars rather well, with accuracy of few precents. This theory leads to the following second-order differential equation:

where p is a constant parameter, 0 ≤ y < 1. In Eq. (1) the values of the parameter n = 0,1 and 2 correspond to the plane-symmetric, cylindrical and spherical self-gravitating configurations of degenerate matter.

Accreting degenerate dwarfs comprise an important class of galactic X-ray sources. This has become evident with the recent discovery of soft and hard X-ray emission from accreting magnetic degenerate dwarfs such as AM Her, AN UMa, and 2A0311-23, and from nonmagnetic or weakly magnetic cataclysmic variables such as SS Cyg, U Gem, EX Hya, and GK Per (for an observational review, see Garmire 1979). Previously known low mass X-ray binaries such as Cyg X-2 and Sco X-1, as well as some of the “bulge” sources, may also be moderate luminosity nonmagnetic degenerate dwarfs (Kylafis and Lamb 1979; Branduardi et al. 1979). Finally, theory predicts (Fabian, Pringle and Rees 1976; Katz 1977; Kylafis and Lamb 1979) an as yet undiscovered class of low luminosity nonmagnetic degenerate dwarf X-ray sources having hard (Tobs ≳ 30–50 keV) spectra.

SS Cygni was found by Joy (1956) to be a spectroscopic binary with an orbital period of about 6-1/2 hours. At minimum light it has mv=12 and is the brightest member of the dwarf nova class of variables. The minimum light spectrum reveals faint, narrow absorption lines of a G- or K-type star along with strong, broad emission lines of hydrogen, helium, and calcium which are produced by an accretion disk surrounding a white dwarf star. Joy’s radial velocities were not very accurate. Nevertheless, he was able to estimate the orbital elements, finding 115 km/s for the absorption line K-velocity and 122 km/s for the emission line K-velocity. In addition, he derived an orbital period of 0276244. Later minimum light observations by Walker and Chincarini (1968) were too few to be able to improve the orbital elements. Kiplinger (1979) refined the emission line radial velocities but was not able to remeasure the faint absorption line spectrum. This paper presents new radial velocity measurements of both the emission and absorption line spectra of SS Cygni at minimum light, and is the first thorough investigation of this star’s radial velocity variations in more than 20 years. The accuracy of the radial velocity curves has been greatly improved. We also find that Joy’s orbital period is in error by nearly two minutes.

An Allegheny parallax series of SS Cyg, consisting of 52 exposures obtained on 15 nights, was recently measured on the PDS microphotometer at the David Dunlap Observatory, and a value of (m.e.) derived for the absolute parallax. This is close to the mean of the two previous discordant measures for this star given in the table below. The weighted mean of the three determinations implies that the absolute magnitude, at quiescent phase, of the star is between 7.0 and 9.0 formally at a 90% confidence level. Recent parallax determinations made at Lick by Vasilevskls et al. (1975) for three other stars, listed below along with the Mt. Wilson value for U Gem, imply even fainter absolute magnitudes.

The writer has undertaken a survey of cataclysmic variables (CV’s) with the image-dissector scanner on the Kitt Peak 2.1-m reflector. A resolution of 7 Å is obtained, and the wavelength interval 3500–5300 Å is covered. Generally only one or two observations of each object are made, with a typical integration time of 10 min. To date, about 100 objects have been observed; most of them are classified in the GCVS as U Gem, Z Cam, or Nl variables or old novae, but a number of irregular variables (types Is, L, ?, or RCB) that appear blue on the Palomar Sky Survey prints have also been included.

Several authors (Begelman et al. 1979; Martin and Rees 1979; Bandwagon 1979) suggest that the precessing relativistic gas jets observed in SS 433 (Abell and Margon 1979) arise from de Sitter and/or Lense-Thirring precession of one member of a close binary system. For a semi-detached system of a few solar masses, we find the mean density of the lobe-filling secondary, ρ̄2 ∽ 3 × 104 gm cm-3. The secondary may thus be a low mass white dwarf, transferring material to the vicinity of its companion at a supercritical rate because of gravitational radiation losses.

Preliminary results of spectroscopic and photometric data for five dwarf novae are presented :

1. 1) V 436 Cen. The orbital period of 0.0669 days was determined from radial velocity variations. The RV half amplitude of the primary, K1. = 159 km/s, implies very small masses of M1 ≲ 0.20 M⊙ and M2 = 0.18 M⊙ for the binary components.

2. 2) Z Cha. Broad emission (Hβ, Hγ, Hδ) and superimposed narrow absorption lines of Hβ-HII, HeI 4471, Cal 4427 and Call K characterize the spectrum during quiescence. Apparently, the cool, optically thin outer disc is seen on the background of a hot continuum, originating from the white dwarf or the inner disc. The RV half amplitude K1 = 87 km/s results in masses of M1 = 1.10 M⊙ and M⊙ = 0.21 M⊙.

3. 3) EX Hya. The RV half amplitude K1 = 68 km/s reveals masses of M1 = 1.4 M⊙ and M2 = 0.19 M⊙ The equivalent widths of the emission lines of H, HeI 4471 and HeII 4686 vary with the phase of the recently detected 67 minute cycle (maximal EW coincides nearly with maximal continuum intensity).

4. 4) 0Y Car is an eclipsing binary with an orbital period of 0.0631 days. The eclipses show strong variations in shape and amplitude in the course of an outburst, similar as those of Z Cha. The observations seem to confirm that the location of the eruption is the central part of the disc which increases in size and luminosity.

5. 5) EK TrA shows periodic superhumps (P = 0.0645 days) during supermaximum, and therefore belongs to the SU UMa sub-group of dwarf novae which are also characterized by a quasi-periodic occurence of super-maxima. The SU UMa sub-group comprises 70% of the ultra-short period cataclysmic binaries, and at least 18% of all dwarf novae.

Linear polarization observations have been made over the past several years on several bright dwarf novae (DN) and related objects (NL). The purpose was to investigate any possible variation that could be related to outburst state (changes in the asymmetry of the disk from max to min), short time scale variability (related to flickering) and variability over the orbital cycle (related to the view of the hot spot or to cyclotron emission as in the AM Her variables). The measurements were made at Kitt Peak (KP), Lowell (L), or Battelle Obs. (B) with either the Dyck (D), Hall/Bowell (HB) or Battelle (B) polarimeters and the system was calibrated in each case by taking 30-40 measurements of the nonpolarized and known polarized standards of Serkowski. The data is summarized in the following table. AE Aqr shows the only variation in mean p between runs while the largest variations on flickering timescales are evident in V Sge, RX And and V426 Oph. VSge and RX And show orbital phase dependent variability. SS Cyg does not show any changes in mean polarization over the outburst cycle although the wavelength dependence of the polarization (U-+R) reverses from max to min.

Absolute spectrophotometric observations of the prototype dwarf nova Z Cam have been obtained at standstill and during the course of an eruption. Optical continuum slopes during the final stages of the rise to maximum appear steeper (bluer) than those observed during decline and at standstill. The Balmer jump is observed to be in shallow absorption during decline and in deeper absorption during the rise to maximum and at standstill. A composite energy distribution constructed for Z Cam at standstill is compared with fluxes derived from steady state (a-raodel) accretion disk models which utilize a grid of stellar energy distributions to synthesize the disk fluxes. The most reasonable accretion disk fit to the observations at standstill suggests a mass transfer rate of ~3 × 1017 g sec-1. The steady state model does not fit the observations of Z Cam during decline, nor does any arbitrary linear combination of stellar energy distributions. The results suggest that the accretion disk of decline is time dependent and exhibits an emission character which differs from that seen at standstill.

We have used a photon-counting array detector at the coudé spectrograph of the Mt. Wilson 2.5-m reflector to follow changes in the near-infrared spectrum of U Geminorum around most of the orbit. A preliminary analysis of two series of exposures shows radial velocity variations in the Na I absorption doublet at λ8183 and λ8195, at the orbital frequency and with the orbital phase appropriate for the M5-type secondary star. A first attempt to detect the K I line at λ7699 was unsuccessful.

Models of U Geminorum fall into two basic types. Both involve a Roche-lobe filling main sequence dwarf orbiting a white dwarf, around which there is an accretion disk. In one type of model there is continuous mass accretion from the inner Lagrange point through the disk to the white dwarf at all times, and the outburst is produced by varying the mass transfer across the inner Lagrange point (Bath et al., 1974). In the second type of model, there is not any accretion from the outer edge of the disk to the white dwarf between outbursts, and matter streaming from the inner Lagrange point accumulates in a torus (Osaki 1974, Paczyński 1978). The outburst is a result of an instability in the disk that suddenly increases the viscosity. A simple diagnostic test of the former type of model is to model the luminosity produced from the accretion disk at minimum light. Following Smak (1976) we adopt a white dwarf mass of 0.9 M⊙, inner and outer radii of the disk of 6.1 × 108 cm and 3.6 × 1010 cm, inclination 67°, and velocity of the stream hitting the disk of 400 km sec-1. Following Wade (1979) we adopt a distance modulus of 4m.4 and a maximum brightness in the visual band of 14m.76. Averaging this radiation over angles and using the velocity information gives a lower limit to the mass transfer rate of 10–10 M⊙ yr-1. Assuming that the disk is optically thick, and taking into account limb darkening in an approximate fashion, yields an expected brightness in the visual band Vdisk ≤ 12m.6. This is two magnitudes too bright. A crude analysis of optically thin disks indicates that stable configurations only exist for disk temperatures close to 1.5×10 °K. This temperature is too low for the accretion rate inferred here, if the disk is optically thin. For this reason the disk must be optically thick. Since the visual magnitude of the disk is too high if it is accreting, we conclude that continuous accretion does not occur at minimum light and that matter accumulates in a torus.

Photometric results by Warner (.1974) and Bailey (1979) are re-analysed to derive geometrical and relative photometric parameters of the system at minimum light. Relative mass of the secondary component is found between μ = 0.1 and 0.2, while its photometric contribution at the bottom of eclipse is between 20 and 40 percent.

By combining spectroscopic data obtained by Vogt (1979) and photometric data obtained by Bailey (1979) and Fabian et al. (1979) it is possible to determine the physical parameters of the southern dwarf nova Z Cha (Ritter, 1979). The resulting values are: mass of the white dwarf primary M1=(0.035±0.06)M⊙, radius of the white dwarf R1=(1.8 10-2)R⊙, mass of the secondary M2=(0.16±0.02)M⊙, radius of the secondary R2=(0.19±0.01)M⊙, inclination i = 748±0.8°. Accordingly the primary conforms to the theoretical M-R relation of white dwarfs and the secondary to the theoretical low mass main sequence. The mass exchange rate derived from the luminosity of the accretion disk and from the hot spot respectively turns out to be Ṁ ≈ 4.10-11 M⊙/year, about 102 times more than is expected from the nuclear evolution of the secondary. On the other hand, angular momentum losses due to gravitational radiation predicts a mass loss rate (Faulkner, 1971) which is almost identical to the one derived from observations. Therefore after the binary pulsar (Taylor et al., 1979), Z Cha seems to be another case presenting strong evidence for the action of gravitational waves.

The shortest known orbital period of a cataclysmic binary with a hydrogen dwarf secondary filling its Roche lobe is about 80 minutes. Theoretically the shortest possible orbital period for such a system is less than 60 minutes. We tried to explain why the periods shorter than 80 minutes are not observed. We estimated the time scale of angular momentum loss of a cataclysmic binary and the resulting mass transfer rate. The minimum orbital period for a given Ṁ is obtained during the transition of the secondary from the Main Sequence onto the Degenerate Dwarf Sequence. Pmin ∝ Ṁ½ Therefore, only those systems can reach low P for which Ṁ is small. This explains why among the shortest period cataclysmic variables there are no novae: presumably their mass transfer rates are too large. It also indicates that “polars” (AM Her-type stars) and SU UMa-type stars should have low Ṁ.

Extensive photoelectric observations of EX Hya were obtained between 1962 and 1976 at different telescopes and observatories. Evaluation of the data reveals a decrease of the orbital period at the rate Ṗ = −5.2 × 10−12. This period change is likely caused by mass transfer with angular momentum exchange between orbital motion and disc rotation on a time scale which is considerably shorter than the corresponding Kelvin – Helmholtz time scale. A rough estimate of the mass transfer rate gives Ṁ ⋍ 2 × 10−9 M⊙/yr. Gravitational radiation losses are negligible in the evolution of EX Hya. Furthermore we report the detection of a periodic modulation of the light-curve with a period of 0.046546447 days and mean amplitudes between 0.2 and 0.4 mag. (“67-minute cycle”). This oscillation remained coherent over the 14 years covered by our observations. The 67-minute variability resembles superhumps seen at supermaxima in SU UMa-type stars with the difference that it shows a 2:3 commensurability to the orbital period. We mention either (i) asynchronous rotation or pulsations of the red component, or (ii) asynchronous rotation of the magnetic primary, or (iii) resonances in the outer regions of the accretion disc as possible sources for this periodicity.

The AAVSO founded in 1911 is the largest organization of variable star observers in the world, with 1100 members in 49 states and 38 countries. AAVSO Headquarters, in Cambridge, MA, receives about 150,000 observations a year from about 400 active observers. Since 1911, over 4 million observations have been compiled. The limiting visual magnitude of observations is about AAVSO’s observing program contains about 2000 variable stars of the following types: Mira (825); semiregular (350); cataclysmic-U Gem, Z Cam, nova, recurrent nova, novalike, symbiotic (200); cepheid (120); R Coronae Borealis (20); RV Tauri (30); nebular (100); flare (15); irregular (140); suspected variables (200). AAVSO has finder charts for most of the stars.

HEAO-1 observations of a large number of cataclysmic variable stars demonstrated that most of these objects are not bright X-ray sources, even during their eruptive phases.

HEAO-1 scanned about 20 dwarf novae while they were undergoing optical outbursts, yet soft X-ray emission (0.18-0.43 keV) was detected only from SS Cyg and U Gem. The 3σ upper limits to the flux from the nondetected objects, several of which were observed by HEAO-1 during more than one outburst, range from 1 to 10% of U Gem’s mean flux of ∽ 1 × 10-10 erg cm-2 s-1. SS Cyg itself was detected during outburst at an intensity 20% of this value.

Many cataclysmic variables have been found to be hard, as well as soft, X-ray sources. Emission from the boundary layer of an optically thick accretion disk extending down to the stellar surface can, at outburst, produce soft X-rays, but the production of hard X-rays from such a disk is difficult to understand. We therefore conjecture that the sources which emit hard X-rays have magnetic fields and are, in general, rotating. We then propose a classification scheme for cataclysmic variables based on the size of the Alfven radius rA relative to the stellar radius R of the degenerate dwarf and the separation α of the binary system. We show that many of the varied characteristics displayed by the cataclysmic variable X-ray sources can be understood in terms of this ordering. We suggest that the AM Her Class (AM Her, AN UMa, W Pup, and 2A0311-23) have R ≪ α ≪ rA , the DQ Her Class (DQ Her, V533 Her, and AE Aqr) have R ≪ rA ≪ α, while the SS Cyg Class (SS Cyg, U Gem, EX Hya, and GK Per) have rA ≲ R ≪ α. Although rA depends on both the magnetic field strength of the degenerate dwarf ana the accretion rate, for comparable rates of accretion the ordering that we propose is essentially one of decreasing magnetic field strength.

Quasi-periodic oscillations on timescales from ~30 sec to 150s have been observed in several dwarf novae during outburst. These oscillations have been qualitatively interpreted as pulsations of the accretion disks in these systems. We explore this suggestion quantitatively by examining the non-radial pulsation properties of viscous accretion disks in the linear, quasi-adiabatic approximation. The weak coupling of the pulsation modes in the radial direction is consistent with the interpretation of these modes as arising in individual disk annuli with periods and eigenfunctions characteristic of the properties of the annulus. The periods of the oscillations, which are periodic in ϕ, z, and t, are shown to be directly proportional to the Keplerian period of the annulus. The existence of a luminosity maximum at some radial position in the disk suggests that the annuli near this position will be the dominant contributors to the observed oscillations. For parameters typical of dwarf novae, the corresponding oscillation periods are ~ 20s, with a range from ~10s to ~ 150s. We suggest that these modes correspond to the observed quasi-periodic oscillations.

Over the past decade it has become increasingly clear that the once commonly held belief that the explosions of dwarf novae are caused by runaway thermonuclear reactions of accreted hydrogen on the surface of a white dwarf is not easily reconcilable with the growing observational data now extending from infra-red to x-ray wavelengths. It cannot fit the disk location of the outburst, the outburst energetics and repetition period (Bath et al. 1974) and the spectrum changes at outburst. The discovery that dwarf novae are semi-detached binaries opened the possibility that the explosions are caused by bursting mass accretion on to the white dwarf (Smak (1971), Bath (1973), Osaki (1974)). This model has now become a powerful tool for investigating the physical and spectroscopic properties of these systems.