ASASSN-16dt and ASASSN-16hg: Promising candidate 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 ≡ 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.

Systematic search for post period-minimum cataclysmic variables: A short review

“…CVs that have passed beyond P min and are evolving back towards longer periods are called the period bouncers. This has been predicted to be the current state of 70% of all CVs [2], however only about two dozens of more or less robust candidates for such period bouncer systems have been identified until now, out of a thousand of known CVs [3,4,5]. Moreover, direct (spectral) evidence for brown-dwarf secondaries exists only for very few CVs; the cleanest example of a spectroscopically confirmed and characterized sub-stellar donor is represented by SDSS J143317.78+101123.3 [6].…”