The light curves of VY Scl stars

The VY Sd stars (sometimes called 'antidwarf novae') are cataclysmic variab les, the light curves of which can be characterized by occasional drops fro m steady high states into low states lasting up to several hundred days. Th ese low states probably result from episodes of low mass transfer from the companion star. The usual thermal-viscous disc instability picture suggests that sufficient mass should be left in the low-state disc to sustain dwarf nova eruptions for some time, even if no matter is transferred. However, s uch eruptions are not seen during VY Sd low states. We present simulations of the evolution of accretion discs in VY Sd systems, including the heating of the disc by radiation from the central white dwarf and boundary layer. The inner region of the accretion disc can be kept fully ionized by a moder ately hot white dwarf, and remains hot even during low states. The transiti on radius between this hot inner region and the cool outer disc moves outwa rd for higher white dwarf temperatures. For a white dwarf temperature T-WD = 20000 K, inside-out outbursts occur during quiescence, starting at the tr ansition radius. When T-WD = 40000 K, the transition radius moves beyond th e region where the disc surface density can build up to the critical value for outbursts, leaving a cool outer region which remains stably in the low state. Thus the observed presence of relatively hot white dwarfs in VY Sd s tars explains why they show no dwarf-nova-type outbursts from the low state .