Discovery of 15 Second Oscillations in Hubble Space Telescope Observations of WZ Sagittae following the 2001 Outburst

We report the discovery of 15 s oscillations in ultraviolet observations of WZ Sagittae obtained with the Hubble Space Telescope approximately 1 month after the peak of the 2001 outburst. This is the earliest detection of oscillations in WZ Sge following an outburst and the first time that a signal near 15 s has been seen to be dominant. The oscillations are quite strong (amplitude ≃5%), but not particularly coherent. In one instance, the oscillation period changed by 0.7 s between successive observations separated by less than 1 hr. We have also found evidence for weaker signals with periods near 6.5 s in some of our data. We discuss the implications of our results for the models that have been proposed to account for the 28 s oscillations seen in quiescence. If the periods of the 15 s oscillations can be identified with the periods of revolution of material rotating about the white dwarf, the mass of the white dwarf must satisfy MWD > 0.71 M☉. The corresponding limit for the 6.5 s signals is MWD > 1.03 M☉.