Although the novalike variable UX UMa strongly resembles the classical nova DQ Her in color, emission spectrum, and optical lightcurve, the properties of the rapid oscillations in both systems are quite different. The oscillations differ in period, amplitude, and phase stability, but most remarkably they differ in the characteristics of the “eclipse related phase shift.” The phase shift in DQ Her is explainable by partial obscuration of the disk during eclipse, together with the idea that the oscillating light does not reach us directly from the white dwarf, but is reflected by the disk. It comes from a rotating UV beam originating near the white dwarf surface, which is reflected better by the backside of the disk than by the front side. We show that the phase shift in UX UMa is explainable by the same model, viewed at a different inclination angle i, if it is assumed that at this value of i reflection from the frontside of the disk is better than from the backside. There may be different ways to accomplish this preference. The results suggest that no retrograde rotation of the white dwarf (or retrogradely rotating nonradial pulsation) is needed to explain UX UMa’s eclipse related phase shift. These phase shifts provide a new (and quite accurate) way to determine a system’s inclination angle. Specific predictions are made for the behaviour of amplitude and phase of the oscillations in other eclipsing systems.

We follow the evolution of an originally widely separated red-giant in orbit with a low mass main sequence star to a cataclysmic binary system. Angular momentum transport via differential rotation leads to a common envelope around the red giant core and the main sequence star. The internal binary separation shrinks by frictional transfer of angular momentum to the extended red giant envelope. This shrinkage continues at nearly constant luminosity until after several hundred years the binary “Roche lobe” cuts into the dense layers of the main sequence star. The envelope will then be lost by a thermal instability. Method and computations for a 5 M⊙ + 1 M⊙ binary are presented elsewhere (Astron. Astrophys. 1979, in press).

Recent theoretical hydrodynamic models show that novae of different speed class can be obtained by varying the CNO enrichment and envelope mass. Recent observations of old nova shells, uncontaminated by disc or stellar emission, indicate that various degrees of CNO enrichment are found in slow novae. Using Prialnik et al’s (1978) finding that the “shut-off” mechanism of novae is fuel exhaustion, i.e. expulsion of most of the hydrogen-rich envelope by means of an optically thick wind, we propose the following unified picture for novae:

1. 1) CNO enrichment and envelope mass Menvel together determine the speed class of a nova.

2. 2) is determined by the accretion mechanism and convective efficiency on the white dwarf surface.

3. 3) Menvel is determined by the nature of the binary system and by envel the accretion rate ṁ.

4. 4) Fast novae tend to have larger and smaller Menvel than slow novae, but CNO-rich slow novae and CNO-poor fast novae are permitted with certain envelope masses. We present below a qualitative diagram of the Menvel, - plane.

High dispersion spectra of WZ Sagittae and Nova Cygni 1978, taken by IUE, have been studied by us.

Of the two spectra of WZ Sagittae in our possession, the long wave region one shows little besides the interstellar Mg II doublet. The more interesting short wavelength region has relatively wide (130-250 km/s) resonance absorption lines sometimes at the center of wider emission. No clear sign of ejection is seen. The study of the profiles of the CIV, NV, OI, and SiIV resonance lines and of Hell 1640 Å indicates:

Seaquist and Palimaka (1977) interpreted the radio observations of this nova according to a “Hubble flow” model, in which all gas was ejected at the same time, and for which the velocity of any point in the shell at a given time is proportional to its radial distance. Their model involving a thick shell contradicts observations made in other parts of the spectrum of this and other novae, so in the present study other possibilities have been examined.

It has been shown by many computations that a nova explosion is triggered by mass accretion onto a white dwarf in a close binary system. Such nova explosions will recur many times in the following way. When a certain amount of hydrogen-rich gas has been accreted on the white dwarf, a hydrogen-shell flash is ignited. Then the hydrogen-rich envelope expands greatly, which, in some cases, grows into a nova explosion. Almost all envelope mass is ejected into space or at least overflows its Roche lobe. After that the mass of the envelope increases again by accretion and the shell flash is ignited again. The period of such recurrence is given by τrec = ΔMH/(dM/dt), where dM/dt and ΔMH are the rate of accretion and the mass contained in the hydrogen-rich envelope at the ignition point.

Observations of recurrent novae (Warner 1976) imply the presence of a red giant (RG) in most, if not all, cases. Giant secondaries in nova systems are otherwise known or suspected only in the peculiar slow novae RR Tel and RT Ser. Bath and Shaviv (1978) showed quantitatively that only a red giant can transfer enough hydrogen-rich mass (~10-5M⊙) to a white dwarf (WD) companion rapidly enough (in ∽30 years) to yield a recurrent nova.

The main features of the spectral evolution of novae consist of: 1. An overall increase in absorption line velocities and emission line widths as a function of time past maximum light. 2. The subsequent appearance of an absorption line system at roughly twice the velocity of the previous absorption line system. 3. Some absorption line systems apparently consist of multiple absorption components at different velocities.

It is suggested that the superhump phenomenon in eruptions of cataclysmic variables like VW Hyi indicates a continuous white light flaring in magnetic flux tubes connecting a magnetic spot on the convective dM secondary and the disk whose Keplerian rotation provides the driving energy by twisting up the magnetic field. Due to the gravitational (and centrifugal) potential difference between the surface of the secondary and the “deeper” disk material a continuous evaporation-condensation mass flow from the secondary to the disk must occur. This can account for the 100-fold increase in mass flow in the disk despite the fact that the ordinary Roche lobe overflow mass flow is not affected, as indicated by the unvaried brightness of the regular accretion spot at the disk. This model interprets the supermaxima eruptions as a consequence of a magnetic cycle on the secondary that periodically immerses the accretion disk in strong magnetic fields. Due to the strong dependence of dipolar field strength on distance from the secondary surface this might explain why the superhump phenomenon is mainly found in the closer dwarf novae. Required field strengths and observed spot sizes are in good agreement with expectations and observations from smaller dM stars like BY Draconis-type stars and flare stars.

Evolution of a mass accreting white dwarf has been computed from the onset of accretion through nova explosion. We have considered a white dwarf of 1.3M⊙ with the accretion rate of 1×10-10M⊙ yr-1. Because the thermal structure during the accretion phase has been fully taken into computation, the mass of the accreted hydrogen-rich envelope and the corresponding temperature distribution in the envelope have been determined. When the hydrogen-rich envelope of mass ΔMH = 1.63 × 10-4M⊙ has been formed, a hydrogen-shell flashes commences. The flashing shell lies midway between the bottom of the envelope and the stellar surface; the mass lying above this shell is 5.7 × 10-5M⊙

Ultraviolet photometry from the OAO-2 satellite supports the white dwarf thermonuclear runaway model for the symbiotic star AG Peg (Gallagher et al. 1979). Massive white dwarfs (M>1.1M⊙) burning hydrogen in a steady state with accretion (Sion et al. 1979) undergo either repetitive hydrogen shell flashes (Ṁ ≲1.03×10-7M⊙/yr), burn hydrogen stably in a steady state with accretion (1.03≲Ṁ≲2.7×10-7M⊙/yr) or evolve into a red giant structure (Ṁ≳2.7×10-7M⊙/yr). The nuclear reactions do not “shut off” between outbursts and mass is not ejected. If these models represent the behavior of the hot component of symbotic variable stars, the high luminosities and masses lead to the possibility that helium shell burning should be important as an additional energy source. The behavior of double shell burning near the surface of a white dwarf of any luminosity has not been explored (Webbink et al. 1978); Models have been constructed to represent such stars having (1) M=1M⊙, Ṁ=1.25×10-7M⊙/yr (L=104L⊙), log Te=5.70 and log R=8.965 and (2) M=1M⊙, Ṁ=1.26×10-8 M⊙/yr (L=103L⊙), log Te=5.52 and log R=8.85. Evolutionary sequences using these initial double shell source models are being constructed.

We compare two hydrodynamic calculations of thermonuclear runaways in material accreted by a 1M⊙ white dwarf of initial luminosity 10−3L⊙. In both cases the CNO abundances are taken to be near solar (ZCNO = 0.014). The only difference between the calculations is that in one sequence of models (seq.B) the additional energy generation due to the interaction between the expanding nova envelope and a close red dwarf companion is allowed for, using a simple model based on that of Paczynski (1976).

Scrutiny of the light curves of the common novae yields important clues concerning both the nature of the nova outburst and the characteristics of the underlying white dwarfs. Ultraviolet and infrared observations have served to make available essentially complete bolometric light curves for several recent novae. These data confirm our earlier prediction that, following maximum, both fast and slow novae experience an epoch (of varying duration) of substantially hydrostatic evolution defined by thermonuclear burning of the residual hydrogen fuel at constant bolometric luminosity. Theoretical studies reveal that the luminosity during this phase of a nova’s evolution is well represented by the Paczynski core mass-luminosity relation for such shell burning configurations involving degenerate stellar cores. This luminosity represents, as well, an increasingly significant fraction of the Eddington luminosity with increasing white dwarf mass.

Recent observations have revealed the existence of infrared brightening in some nova explosions, and its absence in others. These infrared excesses are ascribed to thermal emission from grains which are considered to consist of graphite. Such nova explosions are widely accepted to be triggered by hydrogen shell-flashes on the surface of white dwarfs which accrete matter in close binary systems. As for the hydrogen shell-flash, recently, a general theory applicable even to the case of finite amplitude has been developed. According to this theory, the progress of a shell-flash is determined only by the mass of the white dwarf MWD and the mass of the accreted hydrogen-rich envelope ΔMH.

A summary of the peculiarities of the spectral evolution of the recurrent nova T Cr B are given, and interpreted in terms of an episodic accretion model. On the basis of the marked similarity of U Sco with this nova, it is suggested that the observed maximum will be followed by a secondary maximum, reaching V = 14-15. Such a secondary maximum should be very different for an accretion event (showing a strong continuum) and a thermonuclear outburst (showing a nebular spectrum). If observations of a star at 14th magnitude at the position of U Sco in February 1969 were indeed of a secondary maximum of this nova, the principal maximum probably precedes the secondary by less than 65 days.