SUPERHUMPS IN CATACLYSMIC BINARIES. IX. AL COMAE BERENICES

Abstract

ABSTRACT We report photometry of the 1995 superoutburst of the dwarf nova AL Comae Berenices. The overall eruption light curve was striking, suggestive of two superoutbursts in rapid succession. During the first week of eruption, the light curve sported a period of 81.63 ± 0.07 m. This signal declined quickly in amplitude, and was replaced by a stronger signal at 82.55 ± 0.03 m. The latter bears all the earmarks of a "common superhump," a feature usually seen in SU UMa-type dwarf novae in superoutburst. This superhump endured at least 40 d, with no secular period change. We re-examined the quiescent light curves to search for a stable photometric signal which might signify the true binary period. We found a stable double-humped waveform with a fundamental period of 81.6025 ± 0.0001 m -- the shortest period yet seen among dwarf novae, and probably very nearly the shortest period attainable by any binary star with a hydrogen-rich secondary. In orbital period and quiescent light curve, as well as in the eruption light curve, the star is a virtual twin of WZ Sge. There are also large-amplitude waves with a period in the range of 83-90 m; these "quiescent superhumps" are rarely found in cataclysmic variables, and require an origin somewhat different from that of the common superhumps characteristic of SU UMa stars in eruption. We speculate that they arise from instability at the 2:1 orbital resonance in the accretion disk, and that the secondary has ben whittled down to <0.04 solar mass.