ON THE CERTIFICATION OF AM-CANUM VENATICORUM AS A CATACLYSMIC VARIABLE

We rediscuss the evidence that the peculiar faint blue star AM CVn (= HZ 29) is an ultra-short-period cataclysmic binary. The existence of at least two noncommensurate short periods in the light curve, plus the recent discovery of the class of pulsating DB white dwarfs, suggests a need for reappraisal of the binary star model. Reckoned as a cataclysmic variable, AM CVn is extremely unusual: it never erupts, never changes its spectrum, barely flickers, shows suspiciously blue colors, and fails to reveal a stable photometric period despite titanic efforts to discover it. But many of its properties suggest a similarity to the known DB pulsators. We present new photometry and spectroscopy of AM CVn, and also of five DB pulsators, to illustrate this similarity. It is quite possible that a significant fraction of the total light in AM CVn, perhaps most of it, comes from the photosphere of a pulsating white dwarf. On the other hand, while the original motivations for a binary star model have largely evaporated, several new pieces of evidence in its favor have emerged: the line He II lambda-4686 in emission, variable line profiles, an ultraviolet spectrum with strong heavy-element absorption features, rapid quasi-periodic oscillations, and the discovery of two apparently similar systems with more impressive binary star credentials (V803 Cen and CR Boo). These are not easy to understand in the context of a single-star model. The most severe obstacle to accepting AM CVn as a short-period binary is the instability of the photometric period. We suggest that this can be reconciled with a binary star model by postulating the existence of an eccentric accretion disk precessing around the white dwarf. This implies that the absorption-line profiles should be highly modulated not on the presumed 17 minute binary period but on the disk precession period of approximately 7-12 hr.