SUPERHUMPS IN VY AQUARII

For many years VY Aquarii has been regarded as a charter member of the class of recurrent novae, because it erupted several times, with outburst amplitudes of approximately 8 mag, exceeding those normally attributed to dwarf novae. We present observations obtained both in quiescence and eruption that emphasize the great similarity of the star to well-credentialed dwarf novae like WZ Sagittae. Long-term visual and photographic coverage shows two types of eruptions, differing in brightness and time scale by a factor of approximately 5. A transient wave in the quiescent light curve suggests a period in the range 80-120 min, which could be the orbital period of the underlying binary. Photometry during a long eruption in 1986 showed a large-amplitude modulation (''superhump'') with a period of 92.7 (or possibly 99.1) min, which slowly decreased during the eruption. These are normal signatures of SU Ursae Majoris stars, a subclass of dwarf novae. The superhumps observed in SU UMa stars appear to show a similar degree of period instability, such that P almost-equal-to -(3-10) X 10(-5). Perhaps a more physically significant expression of this is a period-magnitude law: the periods decrease by 0.64 +/- 0.10% per magnitude of decline, at least during the early decline. This is a plausible decrease to expect from the precessing-disk model for the origin of superhumps, assuming the disk to be largest near maximum light.