HS 2331+3905:: The cataclysmic variable that has it all

We report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emission-line object in the Hamburg Quasar Survey. An orbital period of 81.08 min is unambiguously determined from the detection of eclipses in the light curves of HS 2331+3905. A second photometric period is consistently detected at P similar or equal to 83.38 min, similar to2.8% longer than P-orb, which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of similar or equal to5-6 min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12 min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period similar to3.5 h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modeling the far-ultraviolet to infrared spectral energy distribution of HS 2331+3905, we determine a white dwarf temperature of T-eff similar or equal to 10 500 K ( assuming M-wd = 0.6 M-circle dot), close to the ZZCeti instability strip of single white dwarfs. The spectral model implies a distance of d = 90 +/- 15 pc, and a low value for the distance is supported by the large proper motion of the system, mu = 0.14" yr(-1). The non-detection of molecular bands and the low J, H, and K fluxes of HS 2331+3905 make this object a very likely candidate for a brown-dwarf donor.