IP Peg is a U Gem-type dwarf nova with a very high orbital inclination such that the secondary star eclipses the white dwarf primary, accretion disc and hot spot in each orbit. An observing project was set up by ‘The Astronomer’ (TA) magazine and the British Astronomical Association Variable Star Section to make visual and CCD eclipse observations of IP Peg during outbursts in the 1994/95 season.

We present phase-resolved spectroscopy of two dwarf novae, IP Peg and SS Cyg, observed in outburst with the WHT/ISIS spectropolarimeter. Doppler tomograms of several emission lines show peculiar narrow, stationary emission components.

SS Cyg is the brightest known and therefore best studied classical dwarf nova and it was the first one from which X-ray radiation was discovered. SS Cyg is unique because it has been detected at very wide range of energies, from a few eV up to ~ 10 keV (Jones & Watson 1992). It was chosen as the first cataclysmic variable for X-ray observations with the Japanese satellite ASCA. Simultaneous optical photometric and spectroscopic observations were also provided for completeness of the study.

UY Pup is a little studied, frequently out bursting dwarf nova whose brightness varies from V ~ 13.5 mag in outburst to V ~ 15.8 mag in quiescence, which typically lasts ~15…25 d. We observed it spectroscopically at the INT in La Palma on 1989 Febuary 15 and 16 during outburst maximum. In this outburst, the rise lasted at least 3 d and there was a long decline of about 6…9 d. The whole outburst lasted about 18 d (Mattei, private communication).

Although presently classified as a SU UMa-type dwarf nova, WZ Sge is well known as one of the most peculiar objects in that it shows only superoutbursts with exceptional duration and amplitude, and no normal outbursts. Furthermore, on its decline from the 1978 outburst, WZ Sge showed a deep temporal dip. All of these characteristics have puzzled both theoreticians and observers.

The dwarf nova AL Com was photometrically observed during the outburst in 1995 April, which occurred for the first time since 1975. The striking similarity of AL Com to WZ Sge, as demonstrated by the present observation (Fig. 1), provides plenty of material in interpreting the enigmatic nature of WZ Sge-type dwarf novae.

The results of photometric and spectroscopic observations of dwarf novae are presented. The data were obtained during an international program of multiwavelength observations, held in 1986 February at several observatories, of dwarf novae during the first and subsequent days of outburst. During the campaign numerous dwarf novae were monitored in order to catch them in outburst. Preliminary results and analysis of some objects are reported elsewhere. A total of 30 dwarf novae were observed in the northern and southern hemispheres. Among them 37% were caught in outburst, including 10% on the rise to outburst and 17% in decline. Photometric observations were carried out in the UBVRI system and colour indexes were calculated.

We present the preliminary results of a comparison of observed general patterns of quiescent dwarf novae and theoretical predictions of the disk evaporation model. We demonstrate that, even though details still have to be clarified, on the whole there is strong evidence that observations and theory are in agreement.

We attempt to measure the temperature of all the quiescent dwarf novae that have been observed with IUE, by fitting the observed flux distribution of white dwarfs (WDs) with known temperatures. This approach gives results similar to those using much more sophisticated modelling, but allows us to investigate the relationship with system parameters and subclasses.

We present 3-D LTE radiative transfer calculations [1] for H, He and Ca in accretion disks (AD) of dwarf novae in quiescence. The model disk is assumed to be in hydrostatic equilibrium vertically, and to rotate with Keplerian velocities. Calculated emission lines are fitted to phase-averaged, continuum-subtracted spectra of U Gem (Fig. 1) and T Leo (Fig. 2). Up to four parameters of the AD have been fitted: distance D, baryonic number density N, isotropic turbulence Vtu and disk temperature T; the latter two are assumed to be constant throughout the disk. Geometrical parameters are from [2] and [3].

Cataclysmic variables (CVs) are idiosyncratic objects, but progress can be made by studying groups of them. To do this, one must identify samples that are representative of their true properties. Because of the wide variety of physics that occurs in CVs, they have a complex phenomenology in which effects are still being discovered (e.g. permanent superhumps: Patterson & Richman 1991). CV outbursts have their own complex phenomenology (but see Osaki 1995a,b and this volume, for ideas on unification for dwarf novae), so it is unwise to rely on outburst properties to give unbiased samples.

One wants to identify CVs by some property common to all of them. One such property is flickering; another is color excess. CVs are very blue, especially in the ultraviolet, with U – B < −0.46 for 95% of CVs (Bruch & Engel 1994). This color index also defines inclusion in the Palomar-Green (PG) survey (Green, Schmidt & Liebert 1986), and a preliminary list of PG CVs was given by Green et al. (1982). My Ph.D. thesis (Ringwald 1993) completed this work, being the first optically selected complete sample of CVs at high latitude. At the same time, Andy Silber was writing his Ph.D. thesis on an X-ray-selected sample of CVs, from the HEAO-A1 MC-LASS survey (Silber 1992). One might say I was Andy’s optical counterpart.

We present a status report on our ongoing spectrophotometric program to survey as many as possible of the 359 objects in the 751 entry Downes & Shara (1993) catalogue of CVs that are reported as having no published spectroscopy.

We review the various physical processes that could lead to viscosity in accretion discs. A local magnetic dynamo offers the most plausible mechanism and we discuss a simple model in some detail. The dynamo operates even in partially and very weakly ionized discs without much modification.

A magnetic dynamo whose crucial ingredient is the Balbus-Hawley instability (Balbus & Hawley 1991) is a promising candidate for generating accretion disc viscosity. Both semi-quantitative analyses (Tout & Pringle 1992) and simulations (e.g. Brandenberg et al. 1995) suggest that such dynamos can operate in a disc environment. Here we aim to apply simple dynamo ideas to models of dwarf novae, and in particular investigate how dynamo driven discs can produce different values for α in quiescence and in outburst.

The validity of the standard α viscosity prescription in turbulent accretion discs is analyzed. It is found that the α viscosity prescription is no longer valid when the disc is not Keplerian. The analysis is based on simple scaling arguments for fully developed turbulences in accretion discs. An analytical treatment is carried out for geometrically thin, non-Keplerian discs in vertical hydrostatic equilibrium, and is compared with the standard values of the viscosity in the boundary layer region and with the viscosity prescription derived from a general causal equation for the evolution of the stress tensor.

For nearly two decades, our only useful model for the viscosity in accretion disks has been the so-called ‘α-model’. However, it has become clear that the simplest models –in which α is constant – are inadequate to explain the range of behaviours seen in real disks. We show that the properties of steady-state, vertically-averaged models can be determined without any assumptions other than that the disks obey the classical Navier-Stokes equation. These solutions have derived values of α which vary with radius by many orders of magnitude even in small CV disks.

We use two dimensional Smoothed Particle Hydrodynamics simulations to investigate the the Tidal Resonance Instability (TRI) model for superhumps in SU UMa stars.

In the TRI model, superhumps are the signature of an eccentric disk being periodically stressed by the binary system’s rotating tidal field. A slow prograde motion of the disk in the inertial frame is then responsible for the superhump period slightly exceeding the binary orbital period. The disk eccentricity is caused by a 3:1 resonance between orbits in the disk and the orbit of the binary itself. The development of the TRI model is due mainly to Whitehurst (1988), Hirose & Osaki (1990) and Lubow (1991a). Lubow in particular carried out a nonlinear mode analysis of a fluid disk’s response to a weak tidal field and identified the feedback mechanism responsible for disk eccentricity growth.

In this work a 3–D ‘Smoothed Particle Hydrodynamics’ ([1]; [4]; [5]) accretion disc is simulated where particles at its inner edge are dragged by a fast spinning compact central star, as in the case of the intermediate polars. The angular velocity of the central star is twice the orbital angular velocity ωo. This drag can be attributed mainly to viscous interaction in the dense compact star atmosphere, although magnetic coupling may also play a role.

Our simulations have been performed for a system with a primary (a white dwarf) mass M1 = 1.3 M⊙, a secondary mass M2 = 2.2 M⊙, a separation between the components (centre to centre) d12 = 6.33 1011 cm and an orbital period = 1.7 d, the integration time being as long as ≃ 75 d. As in [2] and [3], we have considered quasi-polytropic structures with γ = 1.01. We adopt the term quasi-polytropic since we have in fact solved also an energy equation.

With my collaborators (Ivan Hubeny, NASA/Goddard Space Flight Center; Marcos Diaz, Catherine Grant, and Darren Williams, all of Penn State), I am computing the structure of discs and their corresponding UV spectra for a large grid of choices of the white dwarf mass Mwd and the mass transfer rate Ṁ. In our approach, we divide the problem into a number of parts, each corresponding to a particular FORTRAN-implemented computer program. Program TLUSDISK computes the vertical structure for individual rings in the disc (T, ρ, ne, τ versus Z). The rings are treated independently, as plane-parallel entities without irradiation, obeying the standard expression for Teff as a function of Mwd, Rwd and Ṁ. With this vertical structure, program SYNSPEC computes the detailed local (rest frame) spectrum for each ring. Both specific intensities and angle-averaged fluxes can be computed. Program DISKSYN is used to combine the spectra of individual rings to obtain the integrated or whole-disc spectrum, taking into account Doppler shifts (dependent on the chosen disc inclination) and limb darkening. Finally, program ROTINS is used to smooth the spectrum to an appropriate instrumental resolution. Intermediate results, e.g., from TLUSDISK or SYNSPEC, are stored for repeated use.

The UV emission of accretion disks was synthesized for a grid of optically thick models that cover the high-Ṁ nova-like regime, aiming to quantify the effect of limb darkening in the integrated and radially resolved spectrum of the disk.

We present a further development of the classical eclipse mapping method, which reconstructs the spatial distribution of physical parameters in the accretion disks of cataclysmic variables.