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We report on high‐speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B ≃ 16.2 mag. In comparison with the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ≃ 4 and ≃ 5. Flickering activity is mostly suppressed and the light curve shows the eclipse of a compact white dwarf at the disc centre which contributes ≃ 60 per cent of the total light at 3900–4300 Å. We use measurements of contact phases in the eclipse light curve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q = 0.19 ± 0.03 and an inclination of i = 83 ± 2°. The masses of the component stars are M1 = 0.78 ± 0.06 M⊙ and M2 = 0.15 ± 0.03 M⊙. Our photometric model predicts K1 = 83 ± 12 km s−1 and K2 = 436 ± 11 km s−1. The predicted value of K1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made. The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultrashort period dwarf novae. V2051 Oph is probably a relatively young system, with a secondary star that has not had enough time to evolve out of thermal equilibrium.

We report results of the eclipse mapping analysis of an ensemble of light curves of HT Cas. The fast response of the white dwarf (WD) to the increase in mass transfer rate, the expansion rate of the accretion disc at the same time, and the relative amplitude of the high-frequency flickering indicate that the quiescent disc of HT Has has high viscosity, α ≃ 0.3 − 0.7. This is in marked disagreement with the disc-instability model and implies that the outbursts of HT Cas are caused by bursts of enhanced mass-transfer rate from its donor star.

We report on V and R high‐speed photometry of the dwarf nova EX Draconis (EX Dra) in quiescence and in outburst. The analysis of the outburst light curves indicates that the outbursts do not start in the outer disc regions. The disc expands during the rise to maximum and shrinks during decline and along the following quiescent period. The decrease in brightness at the later stages of the outburst is due to the fading of the light from the inner disc regions. At the end of two outbursts the system was seen to go through a phase of lower brightness, characterized by an out‐of‐eclipse level ≃15 per cent lower than the typical quiescent level and by the fairly symmetric eclipse of a compact source at disc centre with little evidence of a bright spot at disc rim. New eclipse timings were measured from the light curves taken in quiescence and a revised ephemeris was derived. The residuals with respect to the linear ephemeris are well described by a sinusoid of amplitude 1.2 min and period ≃4 yr and are possibly related to a solar‐like magnetic activity cycle in the secondary star. Eclipse phases of the compact central source and of the bright spot were used to derive the geometry of the binary. By constraining the gas stream trajectory to pass through the observed position of the bright spot, we find q=0.72±0.06 and degrees. The binary parameters were estimated by combining the measured mass ratio with the assumption that the secondary star obeys an empirical main‐sequence mass–radius relation. We find M1=0.75±0.15 M⊙ and M2=0.54±0.10 M⊙. The results indicate that the white dwarf at disc centre is surrounded by an extended and variable atmosphere or boundary layer of at least three times its radius and a temperature of T≃28 000 K. The fluxes at mid‐eclipse yield an upper limit to the contribution of the secondary star and lead to a lower limit photometric parallax distance of D=290±80 pc. The fluxes of the secondary star are well‐matched by those of a M0±2 main‐sequence star.

We report the identification of cyclical changes in the orbital period of the eclipsing dwarf novae V2051 Ophiuchi and V4140 Sagitarii. We used sets of white dwarf mid-eclipse timings to construct observed-minus-calculated diagrams covering, respectively, 25 and 16 years of observations. The V2051 Oph data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 22 +/- 2 yr and amplitude 17 +/- 3 s. The statistical significance of this period by an F-test is larger than 99.9 per cent. The V4140 Sgr data present cyclical variations of similar amplitude and period 6.9 +/- 0.3 yr which are statistically significant at the 99.7 per cent level. We derive upper limits for secular period changes of |dP/dt| < 3x10^{-12} and |dP/dt| < 1.8x10^{-11}, respectively for V2051 Oph and V4140 Sgr. We combined our results with those in the literature to construct a diagram of the amplitude versus period of the modulation for a sample of 11 eclipsing cataclysmic variables (CVs). If the cyclical period changes are the consequence of a solar-type magnetic activity cycle in the secondary star, then magnetic activity is a widespread phenomenon in CVs, being equally common among long- and short-period systems. This gives independent evidence that the magnetic field (and activity) of the secondary stars of CVs do not disappear when they become fully convective. We also find that the fractional cycle period changes of the short-period CVs are systematically smaller than those of the long-period CVs.Comment: 9 pages, 4 postscript figures, coded with MNRAS latex style file. To appear in Monthly Notices of the Royal Astronomical Societ

We present and analyze Hubble Space Telescope observations of the eclipsing nova-like cataclysmic variable UX UMa obtained with the Faint Object Spectrograph. Two eclipses each were observed with the G160L grating (covering the ultraviolet waveband) in August of 1994 and with the PRISM (covering the near-ultraviolet to near-infrared) in November of the same year. The system was ∼50% brighter in November than in August, which, if due to a change in the accretion rate, indicates a fairly substantial increase inṀ acc by > ∼ 50%. The eclipse light curves are qualitatively consistent with the gradual occultation of an accretion disk with a radially decreasing temperature distribution. The light curves also exhibit asymmetries about mid-eclipse that are likely due to a bright spot at the disk edge. Bright spot spectra have been constructed by differencing the mean spectra observed at pre-and post-eclipse orbital phases. These difference spectra contain ultraviolet absorption lines and

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When accretion temporarily ceases in the polar AM Her, the emission-line profiles are known to develop several distinct components, whose origin remains poorly understood. The new low-state spectra reported here have a more favorable combination of spectral resolution (R $ 4500), time resolution ($3 minute exposures), and S/ N than earlier work, revealing additional details of the orbital dependence of the line profiles. The central strong feature of H is found to be composed of two components of similar strength, one having K $ 100 km s À1 and phased with the motion of the secondary star, the other having little or no detectable radial velocity variations. We attribute the central line component to gas near the coupling region, perhaps with a contribution from irradiation of the secondary star. The two satellite components have radial velocity offsets of $AE250 km s À1 on either side of the central strong H peak. These satellites most likely arise in large loops of magnetically confined gas near the secondary star due to magnetic activity on the donor star and /or interactions of the magnetic fields of the two stars. Doppler maps show that these two satellite features have concentrations at velocities that match the velocity locations of L4 and L5 in the system.