We review optical observations and theoretical models of the non-magnetic nova-like variables (UX UMa, VY Scl and SW Sex stars). A brief discussion of the classification scheme is followed by a statistical overview of the observed properties. The most important features of each of the sub-classes are then reviewed, concluding with a summary of the theoretical models invoked to understand these systems.

An analysis of the main photometric properties of the nova-like variables AC Cnc, RW Tri and UX UMa, and Nova Her 1934 (DQ Her) is presented. The analysis is based on simultaneous high-speed UBVRI photometry, covering several dozens of orbital cycles evenly distributed over 14 years (1982 ... 1995) for DQ Her, and over 11 years (1985 ... 1995) for the other three stars. All data have been obtained with the 1.25 m telescope at the Crimean Astrophysical Observatory.

We present a progress report on the analysis of time-resolved spectrophotometric observations of the cataclysmic variable V1315 Aql. The spectral range of the data covers the entire Balmer series up to and including the Balmer jump. We aim to use these data to test the steady-state accretion disk model.

We report first results of the HST/FOS time-resolved, low-resolution spectroscopy of the eclipsing cataclysmic variable UX UMa, including multi-wavelength mapping of its accretion disk in the ultraviolet region.

We analyse orbitally-resolved spectra of the Na I doublet in the dwarf nova IP Peg, using the newly developed technique of Roche tomography. We show that Roche tomography can be successfully employed to image the secondary stars in CVs, and that it also serves as an independent aid for determining binary system parameters. Our preliminary results suggest that the mass of the white-dwarf primary is 1.2 M⊙, significantly higher than previous estimates.

The general inability of optical observations of low mass interacting binaries to provide accurate radial velocity measures for the stellar components is a problem for which there is no universally applicable solution. Velocity measurements are handicapped because the accretion disc about the primary often dominates the optical spectrum. Therefore spectroscopists resort to measuring line profile symmetry in individual spectra of the accretion disc in order to provide information on the primary’s motion. However, discs are not axisymmetric structures and the standard approach generally fails because the asymmetry-free inner disc emission is usually dominated by statistical noise.

Our current knowledge of the basic characteristics of CVs, such as their distance and luminosity, are largely based on observations of optically bright (V < 16 mag) systems. This has tended to favour the more luminous members (Mv ~ 7 … 10 mag) lying nearby in the Galactic plane (z < 400 pc;). In order to alleviate this bias, Howell & Szkody (1990) compiled a list of ~90 CVs that were faint enough and were at high galactic latitude to potentially place them well outside the Galactic plane, in the halo. We have obtained infrared J and K band images of a number of the CVs listed by Howell & Szkody (1990) in order to establish whether these systems are truely at large distances, or are instead a population of intrinsically faint objects lying relatively nearby. The distances were determined using Bailey’s method (1982) and incorporated the recent SK calibrations by Ramseyer (1994).

Flickering is a poorly understood phenomenon associated with accretion processes, but this does not suffice to make it interesting. Why then should we bother studying this ‘noise’? Three reasons come to mind: (i) flickering is a fundamental signature of accretion, to the point of being a necessary characteristic (if it doesn’t flicker, it’s not a CV); (ii) energetically, flickering is not a small effect and can contribute up to a few tens of percent of the total luminosity of the system (hence it is often more luminous than the entire secondary star); (iii) flickering is inherently a time-dependent phenomenon and it is hoped that it can provide clues to the nature of the disc viscosity, something that time-independent theory cannot provide.

We describe a new method of orbital light curve decomposition which is applicable to long-term photometry of interacting binaries. This method determines the orbital dependence (including eclipses) of the mean light, secular variability and RMS flickering. We identify the true line of centres of RW Tri and find that the accretion disc is a location of secular variability and a source of flickering.

The variability in the observable light curve of a CV can be attributed to (i) orbital and long-term variations; (ii) flickering, (iii) photon noise, (iv) scintillation (negligible) and (v) any residual noise. Calculating the rms-scatter of the count rates in a light curve as a function of orbital phase, ϕ, through accretion disk eclipse of high inclination CVs – after subtraction of a smoothed light curve to remove orbital and long-term variations, and applying a correction for photon noise – permits one to confine the place of origin of the flickering if its visibility is phase dependent or if eclipses take place (assuming any residual source of noise to be constant in time). The rms-scatter curve is then defined as the function where is the variance due to flickering and the variance due to residual noise.

We report photometric results from 44 runs at 11 observatories during the international campaign ‘TT Ari–94’. No coherent oscillations in the frequency range 10… 900 cycle d−1 are found. The highest peaks in the power spectrum cover the wide range of 28… 139 cycle d−1. Variations occur at a few preferred time-scales rather than at one cycle length, with a possible secular decrease. In the frequency range 90… 900 cycle d−1 the power spectrum obeys a power law with slope γ ranging from 0.8 to 2.6 for different runs.

TT Ari is one of the brightest cataclysmic variables and remains one of the most interesting objects of this class. It exhibits a variety of phenomena observed at time-scales from seconds to months. A recent detailed photometric study of this object and a bibliographical overview may be found in Tremko et al. (1996). Tremko et al. (1992, 1993, 1994) discuss aspects of the TT Ari–88 campaign. Our campaign TT Ari–94 was unprecedented, as the observations were longitude-dispersed, from Japan through Turkmenia, to Europe and the American continent. Moreover, our optical data on October 7 partially overlap with the HST observations obtained by Home & Welsh (1995, private communication). The numerical results of observations obtained during these nights are shown in Table 1.

We present high resolution optical spectroscopy of the cataclysmic variable (CV) V795 Her. The time- and phase- dependent behaviour of the emission source is examined using two-dimensional image analysis and Doppler reconstruction techniques on the orbital profiles. A clear S-wave modulation of a weak emission feature in the Hβ and Hγ line wings (extending to ~1800 km s−1) is present, with complex asymmetric properties.

HST UV observations of V795 Her reveal a strong 2.6-h orbital variation in the prominent UV lines, in contrast to earlier (IUE) evidence of a 4.8-h period. Only the C IV line contains a strong blue-shifted, wind formed absorption component. Several lines exhibit a ‘narrow’ absorption feature near rest velocity which may originate in the disk, and a blue-shifted emission feature which accounts for most of the line profile variability.

Dwarf nova outbursts provide an almost unique opportunity of getting an insight into the nature of viscosity in accretion disks or, within the α- disk approach, of putting some constraints on α. In particular, the strong dependence of the viscous time-scale on viscosity itself permits us to estimate a almost directly from the observed time-scales. In the case of the hot branch of the ∑ — Te relation, the most reliable estimates (αhot) are based on the rate of decline following the dwarf nova outburst. From a comparison with model light curves calculated with different αs one gets: αhot ≈ 0.2(e.g. Smak 1984b). An independent, but much cruder, estimate can be obtained from the widths of normal outbursts, by assuming that the duration of an outburst represents the travel time of an instability wave across the disk. The result is similar: αhot ≈ 0.2 (Gicger 1987).

IUE and optical data were obtained throughout the supercycles of RZ LMi and ER UMa during 1993/94. While most of the spectral and photometric characteristics are consistent with disk changes in systems with a relatively high accretion rate and low inclination, the cause of the high accretion rate in this subset of SU UMa stars is not known.

The ER UMa-type, including ER UMa, V1159 Ori and RZ LMi, is a subgroup of SU UMa-type dwarf novae. Outbursts of these stars are characterized by: (i) the extremely short recurrence time of the superoutburst (~ 40 d in ER UMa and V1159 Ori, ~ 20 d in RZ LMi), (ii) short outbursts with short recurrence times (~ 4 d) between the bright states, and (iii) extremely long duty cycles (~ 0.5). Assuming that the mass transfer rate from the secondary is ten times higher than that of ordinary SU UMa stars, which was invoked by Kato & Kunjaya (1995), and a weak tidal torque in the case of RZ LMi, Osaki (1996) showed that the light curves of these stars can be reproduced by the disk instability model, which does not require mass transfer bursts. These indicate that ER UMa, V1159 Ori and RZ LMi are not on the natural extension of SU UMa-type dwarf novae.

We have observed AK Cnc during the 1995 March superoutburst, by means of differential V – U photometry. The observations were obtained at Las Campanas (Chile), Ouda Station (Kyoto, Japan) and in England. Superhumps with amplitude 0.2 mag appeared at least 4d after maximum. They evolved from a single peak structure, developing a secondary peak during decline, until they reached a double-wave modulation with 0.05 mag amplitude, after 8 d. After removing linear trends in our data, the following ephemeris was found:

This superhump period was confirmed, independently, with our homogeneous set of 462 V magnitudes, obtained at LCO, then adding 63 U data points and finally considering all the dataset, consisting of 789 V + U magnitudes. The superhump period is slightly larger than that reported by Kato (1995) during the 1992 January superoutburst (0.06735(5)d). Our results imply an orbital period Porb = 0.065(2) d, using the relationship given by Howell & Hurst (1994). The eruption duration was ≳ 10 d, and its amplitude > 6 mag. An abrupt and short luminosity decrease occured ≈ 12 d after maximum. Eleven outbursts have been observed since 1992 January, being the minimum reported lag between successive eruptions: 40 d. More details will be given in a future publication.

We have created a database with outburst activity for 269 selected cataclysmic variables. It includes nearly 1000 outburst measurements discovered through 1995 June. The database and a complete paper describing the observations and the sources of information that were consulted are accessible over the Internet.