Discovery of two types of superhumps in WZ Sge-type dwarf nova AL Comae Berenices

A dwarf nova, AL Com, was photometrically observed during an outburst in 1995 April, which occurred for the first time since 1975. The star showed two distinct types of periodic variation: (1) a 0.05666-day variation with decaying amplitudes during the earliest stage of the outburst, and (2) 0.0572-day superhumps which grew similar to 11 d after the onset of the outburst. The present observations are the one first to clearly established the SU UMa-type nature of AL Com. Furthermore, the appearance of these two periods, together with the whole course of the superoutburst, makes AL Com an almost perfect twin of an enigmatic dwarf nova, WZ Sge. We attributed the former period to be the orbital period of this cataclysmic binary. The double-humped profile of this variation seems to preclude an explantion based on an enhanced hot spot by a mass-transfer burst from the secondary, which has been believed to be the best observational evidence of a mass-transfer burst in WZ Sge obtained during its 1978 outburst.