PERIODIC VARIABILITY IN THE OPTICAL COUNTERPART OF X-RAY NOVA MUSCAE 1991

Analysis of over 1000 CCD images of the optical counterpart of the black hole candidate Nova Muscae 1991, taken 3-5 months after the initial outburst, reveal the existence of a luminosity hump of almost-equal-to 0.2 mag which recurs every 10.5 hr. This period presumably either is, or is closely related to, the orbital period, which is thus similar to that of the confirmed black hole system A0620-00. The light-curve shape, the lack of significant color variations, and marginal evidence for a change in the recurrence period suggest that the luminosity humps in Nova Muscae may be related to the superhump phenomenon in the SU UMa subclass of dwarf novae. Other implications of the analogy between soft X-ray transients and dwarf novae are discussed.