Late superhumps in WZ Sge-type dwarf novae

Abstract

We report on the detection of very stable modulations with periods unexpectedly (∼ 0.5%) longer than the superhump periods during the slowly fading stage of WZ Sge-type superoutbursts in three systems: GW Lib, V455 And, and WZ Sge. The periods are naturally explained by assuming that these modulations are superhumps arising from matter near the tidal truncation radius. This finding provides additional support for the hypothetical idea concerning expansion of the accretion disk well beyond the 3:1 orbital resonance in some low mass-ratio systems. Combined with the effect of a 2:1 resonance, we explain the origin of positive period derivatives in certain short-period SU UMa-type dwarf novae. © 2008. Astronomical Society of Japan.