SUPERHUMPS IN CATACLYSMIC BINARIES .9. AL COMAE BERENICES

We report photometry of the 1995 superoutburst of the dwarf nova AL Co mae Berenices. The overall eruption light curve was striking, suggesti ve of two superoutbursts in rapid succession. During the first week of eruption, the light curve sported a period of 81.63+/-0.07 min. This signal declined quickly in amplitude, and was replaced by a stronger s ignal at 82.55+/-0.03 min. The latter bears all the earmarks of a ''co mmon 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 reexamined the quiescent light curves to search for a stable photometric signal which might signify the true binary period , We found a stable double-humped wave with a fundamental period of 81 .6025+/-0.0001 min-the shortest period yet seen among dwarf novae, and probably very nearly the shortest period attainable by any binary sta r with a hydrogen-rich secondary. In orbital period and quiescent ligh t 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 83-90 min; these ''quiescent superhumps'' are rarely found in cataclysmic variables, and require an origin somewhat different fro m that of the common superhumps characteristic of SU UMa stars in erup tion. We speculate that they arise from instability at the 2:1 orbital resonance in the accretion disk, and that the secondary has been whit tled down to <0.04 M(.).