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

Abstract

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 ≃ 83.38 min, ∼ 2.8% 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 ≃5-6 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 ∼ 3.5 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≃10,500 K (assuming M_wd=0.6 M_⊙), close to the ZZ Ceti 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, μ=0.14 arcsec 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.

Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, on observations made at the 1.2m telescope, located at Kryoneri Korinthias, and owned by the National Observatory of Athens, Greece, and with the Isaac Newton Telescope and William Herschel Telescope, which are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.