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

We report the discovery of 15-s oscillations in ultraviolet observations ofWZ Sge obtained with the Hubble Space Telescope approximately one month afterthe peak of the 2001 outburst. This is the earliest detection of oscillationsin WZ Sge following an outburst and the first time that a signal near 15 s hasbeen seen to be dominant. The oscillations are quite strong (amplitude about5%), but not particularly coherent. In one instance, the oscillation periodchanged by 0.7 s between successive observations separated by less than 1 hour.We have also found evidence for weaker signals with periods near 6.5 s in someof our data. We discuss the implications of our results for the models thathave been proposed to account for the 28-s oscillations seen in quiescence. Ifthe periods of the 15-s oscillations can be identified with the periods ofrevolution of material rotating about the white dwarf, the mass of the whitedwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-ssignals is M_WD > 1.03 M_sun.Comment: accepted for publication in ApJ Letters; 13 pages, 4 postscript figures; new version corrects a few typos and matches version that will appear in ApJ