As a consequence of the discovery of the high-frequency variations that define the ZZ Ceti class of variables, observations of them or of new candidates have naturally tended to sample preferentially the period range from 1 or 2 seconds to ~ 30 minutes in unfiltered light. Hence, little is known about variability on time scales > 30 minutes, with the possible exception of some curiosities in the high-frequency literature and in Eggen’s monumental photoelectric UBV observations; a brief summary of some potentially disquieting cases has been given recently by Hesser, Lasker and Neupert (1979). Inasmuch as the existence of unexplained observational discrepancies still admit the possibility that the high-frequency variations may be but one manifestation of the type or time scales of white-dwarf variability, we have measured several ZZ Ceti variables by iris photometry and by visual estimates on the plates of the Harvard collection. Based on an average of 50 observations per star over 55 years, going back to the 1890’s, we find no positive indications of variations in excess of +0.10–0.15 mag for any of the following stars:

PG1159-035 was originally detected as a 14.5 mag. blue object in a survey for QSO candidates (Green 1977), An SIT spectrogram obtained at the Hale 5m telescope at 6Å resolution showed this star to have a very blue continuum with absorption features near λ4686 and λ4650 which were tentatively attributed to Hell and the CIII/CIV complex, respectively. Possible narrow emission components to these lines appear in Figure 1 which shows confirming spectra obtained with the IIDS (at 2.5Å resolution) on the Kitt Peak 2.1m telescope. The blue continuum and possible presence of Hell in emission suggested that this star was perhaps similar to the helium mass-transfer binary AM CVn (HZ 29). For this reason, we put PG1159-035 on our program of high-speed photometry, expecting the star to show “flickering” associated with mass transfer into an accretion disk and possible orbital modulation or eclipses in the period range 10-20 minutes—photometric characteristics similar to those observed in AM CVn.

There are now twelve ZZ Ceti stars known and their properties are summarized by Nather (1978) and McGraw (1977, 1979). They have normal DA spectra and their colors range from a B-V of 0.18 mag to 0.22 mag. This places them on the HR diagram in a region which is an extension to the white dwarfs of the Cepheid instability strip. This would suggest that the cause of their pulsation could be analogous to the κ and γ mechanisms of the Cepheids except for the fact that the periods of the ZZ Ceti stars range from 200 to 1200 seconds, three orders of magnitude larger than the radial pulsation periods for white dwarfs.

A white dwarf model with M=.6 M⊙, Te=12000K, and L=1.2×1031 erg sec-1 provided by A.N. Cox has been tested for linear stability of radial oscillations. The radial mode instability first reported for this model by Cox, et. al (1979) has been confirmed. The growth rates obtained are comparable to the rates found by Cox. A sequence of ℓ=2 g-modes has also been found to be unstable. The e-folding times range from around 1011 periods for a 137 second mode (1 radial node) to less than 100 periods for a 629 second mode (17 nodes). It is likely that the latter rate is too high because the eigenfunction has been forced to vanish at the non-zero inner radius of the model, at which the Brunt-Väisäla frequency is barely less than the mode frequency.

By means of a simple analysis, it is shown that the combination of long periods and relatively high effective temperatures in ZZ Ceti variables may be made consistent with each other if the driving region for instability occurs at a temperature of (1–2) × 105K. Since this temperature does not coincide with the zone of He+ ionization, the driving mechanism is then not the same as for the Cepheid variables and the linear extension of the Cepheid strip down to the ZZ Ceti white dwarfs may be regarded as a coincidence. A Stelllngwerf “bump” type mechanism may be more appropriate for the latter. However, it is also shown that the short period modes computed by Dziembowski and found to be unstable, are consistent with the Cepheid mechanism. A special plea is made for further opacity calculations at temperatures of ~105 K and densities within a few orders of magnitude of unity — a ρ-T region of considerable interest for the envelopes of ZZ Ceti variables.

In previous work (Papaloizou and Pringle, 1978), we investigated analytically the low frequency spectrum of a rotating pulsating star, taking the effects of rotation fully into account. In that case we found that not only are the g-modes substantially modified by the rotation, but also that a new set of modes appear (the r modes) which are similar to Rossby waves in the Earth’s atmosphere and oceans.

Both SS Cyg and U Gem show enhanced soft X-ray emission (0.1–0.5 keV) during optical outbursts. In extended HEAO-1 pointings, it was discovered that a large fraction of this emission is pulsed (see Fig. 1). This confirms the general expectation that high energy pulsations are the ultimate source of the optical pulsations observed during outbursts of dwarf novae. For SS Cyg the average soft X-ray pulsation amplitude is 30%; this is to be contrasted with the upper limit of 10% to a hard X-ray (2–25 keV) pulsed component for the same observation (Swank 1979). Thus the origin of the oscillation is in soft X-rays, and the direct implication of such a large pulsed amplitude is that the oscillation itself must be an intrinsic part of the X-ray production mechanism.

With a high speed photometric system we have made extensive observation of the dwarf nova AH Her. We have detected rapid oscillations on two consecutive nights during the recovery portion of the light curve of a typical outburst during the spring of 1979. The periods and the sense of the change of period are consistent with previous observations of rapid oscillations of AH Her. On another occasion we detected quasi-periodic oscillations during the rising portion of an outburst. These are similar in appearance to quasi-periodic oscillations found in other cataclysmic variables in that they appear with high statistical significance in the autocorrelation function but are not apparent in power spectra (Robinson and Nather 1979). However they are different from those reported by Robinson and Nather in the sense that they have the same order of period as the periodic rapid oscillations typical of AH Her. These results will be presented together with a discussion of their Implications.

Theoretical continua (3000 ≲ λ (Å) ≲ 20000), UBV colours and Balmer absorption line profiles (Hα – Hδ) have been computed for the optically thick parts of steady-state accretion discs around white dwarfs as a function of mass flow rate, mass of white dwarf and inclination angle. The uncertainties due to disc z-structure and outer radius and viscosity are considered.

The theoretical results for continua and UBV colours are compared with the theoretical results of Schwarzenberg-Czerny and Rózyczka (1977) and Herter et al. (1979) and with results obtained using black bodies instead of stellar atmospheres (e.g. Lynden-Bell 1969). Where models can be directly compared the quantitative agreement is good.

Kylafis and Lamb (1979, hereafter KL) have recently completed a comprehensive study of X-ray emission from accreting nonmagnetic degenerate dwarfs. Their calculations assumed a common temperature for the electrons and the ions, and neglected electron conduction. However, when Compton cooling of the X-ray emission region becomes important at high masses, electron conduction begins to play a role (Kylafis 1978).

DQ Her is one of the best studied Cataclysmic Variables. It is a classical nova which erupted in 1934 (Nova Herculis). Its optical lightcurve does not repeat accurately from one binary cycle to the next, but usually looks roughly like it does in Fig. 1. The small hump and small eclipse-asymmetry are probably both caused by the hot spot at the outer rim of the disk. The eclipse depth of ~90% argues for an inclination angle near 90°.

For isolated stars, identification as a white dwarf may be effected in several ways. The fundamental property of abnormally low luminosity can be detected through direct measurement of trigonometric parallax or indirectly through large proper motion (accompanied by appropriate photometric properties). The presence of greatly pressure broadened absorption lines is another unambiguous criterion. Rapid light oscillations of the kind reviewed by Robinson are another hallmark of a select group of white dwarfs. Any or all of these criteria may be used to classify a star as a white dwarf and in general can be applied to members of wide binary systems.

Three major modes of interaction in binary systems are identified, corresponding to (I) physical contact, (II) tidal mass loss from a star with a radiative envelope, and (III) tidal mass loss from a star with a convectlve envelope. The qualitative evolution of binary systems in each of these cases is outlined, leading to the formation of single white dwarfs, long-period remnants of quasi-conservative mass transfer, or short-period cataclysmic binaries. The origin of short-period double white dwarfs in a second phase of mass transfer is outlined. Implications for the frequency of different systems of these types, and of planetary nebulae with close binary nuclei, are discussed.

V Sge is a short period, double-line spectroscopic and eclipsing close binary which is also intrinsically variable through a range of at most 3 mag in visual and photographic light. Almost all modern insight is founded upon the interpretation by Herbig, et al. (1965), who developed the evidence for the binary nature of the star, evaluated the nearly 0.5 day Keplerian period, studied the short-term Doppler and intensity variations of the spectrum, and obtained a limiting representation of the intrinsically noisy ultraviolet light curve. The orbital plane view of the binary, as developed by these authors, shows patchy distributions of 0 IIIf1 and 0 VI emissions and mass loss from the system as a whole.

The proper-motion star G61-29 (Giclas et al 1971) was discovered to have an emission-line spectrum consisting entirely of helium by Burbidge and Strittmatter (1971), who noted that the very broad lines (ca. 32 Å at λ4471) seemed to have variable profiles. Warner (1972) suggested that the star might be a close binary system, with the emission lines arising from an accretion disk surrounding a white dwarf, and searched for evidence of binary motion using high speed photometry. He suggested a tentative orbital period of 6h 16m, which subsequent observations were unable to confirm. Smak (1975) studied the star spectroscopically and agreed with Warner’s accretion disk hypothesis based on his analysis of the emission line profiles, but was also unable to derive evidence of orbital motion from his spectra, which were exposed for about 40 minutes each. Greenstein et al.(1977) also observed profile variations in spectra with shorter exposures, but evidently did not search for periodicities.

The short period eclipsing binary system and recurrent nova WZ Sge (1913, 1946, 1978) was observed spectroscopically during its renewed, December 1978 outburst. Absorption line radial velocities and Hα emission profiles suggest a clrcumblnary gaseous disk. A schematic model is presented to explain both the observed radial velocities and the photometric properties of WZ Sge during outburst. The model also accounts for ‘superhump’ phenomena in SU UMa type dwarf novae.

The unusual variable star CH Cygni was an object of numerous investigations over past ten years. The aim of the present paper is to give a summary of the most important observational data and to discuss briefly a double star model of CH Cygni which is in general accordance with observational facts.

This paper presents some preliminary results of work aimed at producing model atmospheres for the steady-state, constant luminosity continuous ejection model of classical novae. The results are generally, applicable to any object with an outflowing atmosphere driven predominantly by radiation pressure in an optically thick continuum. In particular it is aimed at removing the limitations of assuming blackbody photospheric boundary conditions and strict LTE and the problems associated with their definition.

Questions concerning the origin and evolution of cataclysmic variables continue to be the subject of considerable inquiry and debate. Significant boundary conditions upon theoretical models may be imposed by our increasing knowledge of the characteristics of specific systems. It is the purpose of this contribution to argue that, within the framework of the thermonuclear runaway model, the classical novae are most reasonably interpreted as systems characterized by relatively massive white dwarf components.

Some aspects of the observational data on cataclysmic binaries are discussed and some possible correlations between type of behaviour and binary period are noted. A gap between 2 and 3 hours in the histogram of binary periods is estimated to be real. A numerical procedure for following the evolution of Roche-lobe-fiUing stars using simplified equations is described. This procedure Is applied to white/red dwarf binaries for a variety of initial conditions, and of mass loss and angular momentum loss mechanisms. The results of these calculations, in which we ignore the short timescale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The results are discussed in connection with cataclysmic binaries. The clustering in period below 2 hours is Interpreted in terms of evolution following the hydrodynamical mode, and it is suggested that such systems contain low mass white dwarfs as well as low mass secondaries. These may be the most common type of cataclysmic binary. A possible explanation of the clustering of classical novae systems to binary periods of 3 to 5 hours is mentioned, and evolutionary scenarios for cataclysmic binaries are outlined. We suggest, following Ritter and Webbink, that the short period systems (≲ 2 hrs) arise mainly from late Case B mass transfer in the original binary (original primary mass 1.5 to 3M⊙) and the longer period systems arise mainly from Case C mass transfer.

Full text to be published in Monthly Notices of the Royal Astronomical Society.