On the certification of AM Canum Venaticorum as a cataclysmic variable

Abstract

The evidence that the peculiar faint blue star AM CVn (= HZ 29) is an ultrashort-period cataclysmic binary is reexamined. 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. New photometry and spectroscopy of AM CVn and of five DB pulsators are presented to illustrate the similarity between AM CVn and the known DB pulsators. It is proposed that a significant fraction of the total light in AM CVn, perhaps most of it, comes from the photosphere of a pulsating white dwarf. The most severe obstacle to accepting AM CVn as a short-period binary is the instability of the photometric period. It is suggested that this can be reconciled with a binary star model by postulating the existence of an eccentric accretion disk processing around the white dwarf. This implies that the absorption-line profiles should be highly modulated not on the presumed 17-min binary period but on the disk precession period of about 7-12 hr.