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Abstract:

I present a variation on Osaki's tidal-thermal-instability model for SU UMa behaviour. I suggest that in systems with the lowest mass ratios, the angular-momentum dissipation in an eccentric disc is unable to sustain the disc on the hot side of the thermal instability. This decoupling of the tidal and thermal instabilities in low-q systems allows a better explanation of the 'echo' outbursts of WZ Sge systems and the short supercycles of ER UMa stars. The idea might also apply to the soft X-ray transients.

Abstract:

We investigate the effect of orbital decay caused by nuclear expansion of a (sub)giant star in synchronous binary system. We compare this effect with the presence of a magnetic stellar wind and show that the additional transfer of orbital angular momentum into spin angular momentum is relatively important - especially since it has been shown that the effect of magnetic braking saturates at short orbital periods.

Abstract:

The binding energy of the envelope to the core of a donor star will determine the outcome of a binary evolving through a common envelope (CE) and spiral-in phase. This binding energy is expressed by a parameter lambda which depends strongly on the evolutionary stage (i.e. stellar radius). Following our earlier publication, we present the lambda-parameter of stars with masses 11 - 100 M..

Abstract:

We study the accretion efficiency of neutron stars in long-period binaries (P greater than or similar to 20(d)) which accrete from a giant companion. Using a-disc models and taking into account the effect of irradiation of the accretion disc by the central accretion light source we derive explicit expressions for the duty cycle d and the accretion efficiency 77 in terms of the parameters of the binary system and the disc instability limit cycle. We show that the absence of millisecond pulsars in wide binaries with circular orbits and periods P greater than or similar to 200(d) can be understood as a consequence of the disc instability if the duration of the quiescent phase between two subsequent outbursts is at least a few decades.

Abstract:

We combine AAVSO and VSS/RASNZ optical and Extreme Ultraviolet Explorer EUV light curves of dwarf novae in outburst to place constraints on the nature of dwarf nova outbursts. From the observed optical-EUV time delays of approximate to 0.75-1.5 days, we show that the propagation velocity of the dwarf nova instability heating wave is 3 km s(-1).