Orbital periods of cataclysmic variables identified by the SDSS

Abstract

We present VLT spectroscopy and NTT photometry of the faint cataclysmic binary SDSS J003941.06 + 005427.5. This object shows triple-peaked Hα emission with all three peaks variable in both strength and velocity. We measure an orbital period of 91.395 ± 0.093 min from the velocity variations of the wings of the Hα emission line. Using the GALEX and SDSS photometry of this object, we determine a white dwarf temperature of 15 000 K and a very late (≳L2) spectral type for the companion star. These measurements, plus the relatively long orbital period, suggest that SDSS J003941.06 + 005427.5 may be a post-bounce cataclysmic variable. Doppler maps of the Hα and He I 6678 Å emission features show an accretion disc with a non-uniform brightness and departures from Keplerian flow. The third emission peak is detected only in Hα and at a relatively low velocity amplitude of 202 ± 3 km s-1. We are unable to explain this emission as arising from either the white dwarf, the secondary star, or the accretion disc. We tentatively attribute this mysterious central peak to a coronal loop anchored at the secondary star. If confirmed, this would be the first example of a slingshot prominence in a CV with a low mass-transfer rate and/or a fully convective secondary star.