Late Superhumps in WZ Sge-Type Dwarf Novae

TL;DR

This paper reports the detection of stable superhump modulations longer than typical periods in WZ Sge-type dwarf novae, supporting the idea of disk expansion beyond the 3:1 resonance and explaining positive period derivatives.

Contribution

It provides observational evidence of superhumps from matter near the tidal truncation radius, supporting disk expansion beyond the 3:1 resonance in low mass-ratio systems.

Findings

Detection of superhumps ~0.5% longer than superhump periods

Superhumps originate near the tidal truncation radius

Explains positive period derivatives in short-period dwarf novae

Abstract

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