ASASSN-16dt and ASASSN-16hg: Promising Candidates for a Period Bouncer

TL;DR

This study analyzes optical light curves of two WZ Sge-type dwarf novae, identifying features indicative of very low mass ratios and suggesting they are promising candidates for period bouncers, which are post-period minimum cataclysmic variables.

Contribution

It provides detailed observational evidence linking light curve features to low mass ratios and proposes ASASSN-16dt and possibly ASASSN-16hg as strong period bouncer candidates.

Findings

ASASSN-16dt has a mass ratio of 0.036(2).

Light curve dips are caused by slow evolution of tidal instability.

Properties match those expected for period bouncers.

Abstract

We present optical photometry of superoutbursts in 2016 of two WZ Sge-type dwarf novae (DNe), ASASSN-16dt and ASASSN-16hg. Their light curves showed a dip in brightness between the first plateau stage with no ordinary superhumps (or early superhumps) and the second plateau stage with ordinary superhumps. We find that the dip is produced by slow evolution of the 3:1 resonance tidal instability and that it would be likely observed in low mass-ratio objects. The estimated mass ratio ($q \equiv M_{2}/M_{1}$) from the period of developing (stage A) superhumps (0.06420(3) d) was 0.036(2) in ASASSN-16dt. Additionally, its superoutburst has many properties similar to those in other low-$q$ WZ Sge-type DNe: long-lasting stage A superhumps, small superhump amplitudes, long delay of ordinary superhump appearance, and slow decline rate in the plateau stage with superhumps. The very small mass ratio and observational characteristics suggest that this system is one of the best candidates for a period bouncer -- a binary accounting for the missing population of post-period minimum cataclysmic variables. Although it is not clearly verified due to the lack of detection of stage A superhumps, ASASSN-16hg might be a possible candidate for a period bouncer on the basis of the morphology of its light curves and the small superhump amplitudes. Many outburst properties of period-bouncer candidates would originate from the small tidal effects by their secondary stars.