A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

A B S T R A C T We present time-resolved spectroscopy and photom etry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey D ata Release 4. The orbital period determ ined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dom inated to 90% by emission from the white dwarf. The spectrum can be successfully reproduced by a three-com ponent model (white dwarf, disc, sec­ ondary) with Twd = 12 500K for a fixed logg = 8.0, d = 170pc, and a spectral type of the secondary later than M8. The mass transfer rate corresponding to the optical luminosity of the accretion disc is very low, ~ 1.7 x 10-13 M0 y r- 1 . Optical photom ­ etry reveals a coherent variability at 641s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photom etric variation with a period of either 320 min or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photom etric signals have been found in the CVs SDSS J123813.73-033933.0, SDSS J204817.85-061044.8, G W Lib and FSA ur, but so far no working model for this behaviour is available. K e y w ord s: Stars: individual: SDSS J133941.11+484727.5 - novae, cataclysmic vari­ ables - stars: oscillations - white dwarfs