Transitions to and from stable discs in cataclysmic variable stars

The observational properties of Z Cam stars, nova-like variables and nova remnants are discussed in terms of the disc instability model. The effects of irradiation on the secondary star, giving rise to high and low states of mass transfer, and on the inner disc, suppressing or greatly reducing the disc instability, are used to explain the light curves of VY Scl stars. Irradiation may also play a part in producing low amplitude modulations in brightness of nova-likes on time scales of tens of days.     

Recurrent novae

A brief review of recurrent novae studies which have been undertaken in South Africa during the last ten years is given. These works have substantially altered our view of these objects as distinct sub-classes of cataclysmic variable stars. Observational results which led to new developments in our understanding of the recurrent novae are reviewed in some detail. Observations of V1017 Sgr, which had been suggested as a recurrent nova, are also mentioned.     

激变变星的辐射模型和多色测光的对比研究

基于黑体模型，本文对激变变星各成分的辐射作了定量的分析，并与观测作了比较。结果表明，简化模型与大多数观测拟合得较好。通过这个模型，我们可以大致看出激变变星不同成分在不同的波段对辐射贡献的相对比例，同时也为我们以前的统计结果给出了一些理论依据。     

The standstill luminosity in Z Cam systems

We consider accretion discs in close binary systems. We show that the heating of a disc at the impact point of the accretion stream contributes significantly to the local energy budget at its outer edge. As a result, the thermal balance relation between local accretion rate and surface density (the 'S-curve') changes; the critical mass transfer rate above which no dwarf nova outbursts occur can be up to 40 per cent smaller than without impact heating. Standstills in Z Cam systems thus occur at smaller mass transfer rates than otherwise expected, and are fainter than the peak luminosity during the dwarf nova phase as a result.     

Magnetic activity in accretion disc boundary layers

We use three-dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high-density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle , the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.     

The orbital period and variability of the dwarf nova ES Draconis

A radial velocity study of the cataclysmic variable ES Dra (PG 1524+622) is presented. ES Dra is found to have an orbital period of 0.17660 ± 0.00006 day (4.2384 ± 0.0014 h). The mass-losing secondary star of ES Dra is detectable in the spectrum, and it has a spectral type of M2 ± 1. From this, we estimate the absolute magnitude of ES Dra during our spectroscopic observations to have been M_R = 6.5 ± 0.5, and its distance to be 720 ± 150 pc. The long-term light curve of ES Dra compiled by the American Association of Variable Star Observers (AAVSO) shows that ES Dra is a Z Cam star, which between 1995 and 2009 spent most of its time in standstill.     

Multi‐epoch time‐resolved photometry of the eclipsing polar CSS081231:071126+440405

The eclipsing polar CSS081231:071126+440405 turned bright (V_max ∼ 14.5) in late 2008 and was subsequently observed intensively with small and medium‐sized telescopes. A homogeneous analysis of this comprehensive dataset comprising 109 eclipse epochs is presented and a linear ephemeris covering the five years of observations, about 24000 orbital cycles, is derived. Formally this sets rather tight constraints on the mass of a hypothetical circumbinary planet, M_pl ≤ 2 M_Jup. This preliminary result needs consolidation by long‐term monitoring of the source. The eclipse lasts 433.08 ± 0.65 s, and the orbital inclination is found to be i = 79.3°–83.7°. The centre of the bright phase displays accretion‐rate dependent azimuthal shifts. No accretion geometry is found that explains all observational constraints, suggesting a complex accretion geometry with possible pole switches and a likely non‐dipolar field geometry. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Zum optischen Langzeitverhalten von AM Herculis I. Das Beobachtungsmaterial,die Langzeitlichtkurve und der bahninduzierte Lichtwechsel

For the variable binary AM Herculis, which is the proto-type of a subgroup of cataclysmic stars, results from the study of long-term behaviour are given. The following part of the publication concerns the observation series, the long-term light curve and the orbital light changes. On the base of 1275 photographic observations in B, V and U, most of B, covering the time interval between the years 1982 and 1991 new statements can be made about the common photometric behaviour of the star, its low and high states and the mutual correlations between them, the individual influences of the orbital light changes superimposed to the total brighhtness and last but not least about the colour behaviour of the star within these individual occurrences and the whole time interval, which is object of the second part of this study.     

Doppler tomography of cataclysmic variables

The study of cataclysmic variables (CVs), and in particular of the evolution of their accretion discs throughout their different brightness states, has benefited largely from the use of indirect imaging techniques. I report on the latest results obtained from Doppler tomography of CVs concentrating mainly on results published since the 2000 Astrotomography meeting in Brussels. Emphasis is given to the spiral structures found in the accretion discs of some CVs, to the evolution of these structures throughout quiescence and outburst, and to our search for them in more systems. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)