  We present time-series VLT spectroscopy and NTT photometry of the cataclysmic
variable SDSS J220553.98+115553.7, which contains a pulsating white dwarf. We
determine a spectroscopic orbital period of 82.825 +/- 0.089 min from velocity
measurements of the Halpha emission line. A period analysis of the light curves
reveals a dominant periodicity at 44.779 +/- 0.038 min which is not related to
the spectroscopic period. However, the light curves do not exhibit a variation
at any frequency which is attributable to GW Lib type pulsations, to a
detection limit of 5 mmag. This non-detection is in contrast to previous
studies which have found three pulsation frequencies with amplitudes of 9 to 11
mmag at optical wavelengths. Destructive interference and changes to the
thermal properties of the driving layer in direct response to accretion can be
ruled out as causes of the non-detection. Alternatively, it is feasible that
the object has cooled out of the instability strip since a previous
(unobserved) dwarf nova superoutburst. This would be the first time this
behaviour has been seen in a cataclysmic variable pulsator. Another possibility
is that changes in the surface characteristics, possibly induced by accretion
phenomena, have modified the surface visibility of the pulsation modes. Further
observations, particularly improved constraints on the timescale for changes in
the mode spectrum, are needed to distinguish among possible explanations.
