Absolute magnitudes of cataclysmic variables

Absolute magnitudes Mv of the accretion discs (with allowance for effects of inclination) of stars in the principal classes of cataclysmic variable stars are derived from a variety of techniques. For dwarf novae, whose distances are found mostly from infrared observations of their secondaries, a tight relationship is found between Mv(max) at maximum of outburst and orbital period: Mv(max)=5.64 – 0.259 P(hr). Use of this equation provides Mv(min) at minimum light for further dwarf novae without known distances. For many of these stars it is possible also to derive Mv(mean), averaged over the outburst cycle. Analysis of this extensive data set discloses that Mv(min) and Mv(mean) are functions of orbital period P and mean time Tn between normal outbursts. Tn is shorter for the brighter systems. The Z Cam stars have Mv(mean) close to the theoretical value for stable (rather than episodic) mass transfer through accretion discs. Above that critical level all observed discs are stable. An amplitude – Tn (Kukarkin–Parenago) relationship is found to exist for the dwarf novae. Together with the above results it leads, through a simple model for disc instabilities, to a predicted relationship between duration of dwarf nova outbursts and P which is verified by observation. Absolute magnitudes of classical novae at minimum light, derived from application of the Mv(max) – rate of decline relationship, average Mv=4.4 which is close to the luminosity of dwarf novae during outburst. Nova-like variables have similar luminosities. Together these results suggest the possibility of an upper limit to the rate of mass transfer in an accretion disc. A scenario is given in which recent suggestions on cyclical evolution through classical novae, dwarf novae and nova-like variables are shown to agree qualitatively with the systematics found here. A relatively short interval ( ~ 103 yr) between successive nova outbursts appears likely. The Appendix lists cataclysmic variables which are nearer than 300 pc, which should be targets for trigonometrical parallaxes.