Two new accreting, pulsating white dwarfs: SDSS J1457+51 and BW Sculptoris

We report the discovery of rapid periodic signals in the light curves of two cataclysmic variables with prominent white dwarf components in their spectra, SDSS J1457+51 and BW Sculptoris. These stars therefore appear to be new members of the GW Librae class of variable stars, in which the fast periodic (and non-commensurate with the orbital period) signals are believed to arise from non-radial pulsations in the underlying white dwarf. The power spectra of both stars show complex signals with primary periods near 10 and 20 min, respectively. These signals change in frequency by a few per cent on a time-scale of weeks or less, and probably contain an internal fine structure unresolved by our observations. We also detect double-humped waves signifying the underlying orbital periods, near 78 min for both stars. In addition, BW Sculptoris shows a transient but powerful signal with a period near 87 min, a quiescent superhump. The similar to 11 per cent excess over the orbital period is difficult to understand, and may arise from an eccentric instability near the 2:1 resonance in the accretion disc.