2008MNRAS.388..709S

We present time-series Very Large Telescope (VLT) spectroscopy and New Technology Telescope (NTT) photometry of the cataclysmic variable SDSSJ220553.98+115553.7, which contains a pulsating white dwarf. We determine a spectroscopic orbital period of P_orb= 82.825±0.089min from velocity measurements of the Hα emission line. A period analysis of the light curves reveals a dominant periodicity at P_phot= 44.779±0.038min which is not related to the spectroscopic period. However, the light curves do not exhibit a variation at any frequency which is attributable to GWLib-type pulsations, to a detection limit of 5mmag. This non-detection is in contrast to previous studies which have found three pulsation frequencies with amplitudes of 9-11mmag 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 time-scale for changes in the mode spectrum, are needed to distinguish among possible explanations.