  We present a time series analysis of Hubble Space Telescope observations of
WZ Sge obtained in 2001 September, October, November and December as WZ Sge
declined from its 2001 July superoutburst. Previous analysis of these data
showed the temperature of the white dwarf decreased from ~29,000 K to ~18,000
K. In this study we binned the spectra over wavelength to yield ultraviolet
light curves at each epoch that were then analyzed for the presence of the
well-known 27.87 s and 28.96 s oscillations. We detect the 29 s periodicity at
all four epochs, but the 28 s periodicity is absent. The origin of these
oscillations has been debated since their discovery in the 1970s and competing
hypotheses are based on either white dwarf non-radial g-mode pulsations or
magnetically-channelled accretion onto a rotating white dwarf. By analogy with
the ZZ Ceti stars, we argue that the non-radial g-mode pulsation model demands
a strong dependence of pulse period on the white dwarf's temperature. However,
these observations show the 29 s oscillation is independent of the white
dwarf's temperature. Thus we reject the white dwarf non-radial g-mode pulsation
hypothesis as the sole origin of the oscillations. It remains unclear if
magnetically-funnelled accretion onto a rapidly rotating white dwarf (or belt
on the white dwarf) is responsible for producing the oscillations. We also
report the detection of a QPO with period ~18 s in the September light curve.
The amplitudes of the 29 s oscillation and the QPO vary erratically on short
timescales and are not correlated with the mean system brightness nor with each
other.
