A survey for post common-envelope binary stars using GALEX and SDSS photometry

We report the first results of our programme to obtain multi-epoch radial velocity measurements of stars with a strong far-UV excess to identify post common-envelope binaries (PCEBs). The targets have been identified using optical photometry from SDSS DR4, ultraviolet photometry from GALEX GR2 and proper motion information from SDSS DR5. We have obtained spectra at two or more epochs for 36 targets. Three of our targets show large radial velocity shifts (> 50 km s) on a timescale of hours or days and are almost certainly PCEBs. For one of these targets (SDSS J104234.77+644205.4) we have obtained further spectroscopy to confirm that this is a PCEB with an orbital period of 4.74h and semi-amplitude K = 165 km s. Two targets are rapidly rotating K-dwarfs which appear to show small radial velocity shifts and have strong Ca II H+K emission lines. These may be wind-induced rapidly rotating (WIRRing) stars. These results show that we can use GALEX and SDSS photometry to identify PCEBs that cannot be identified using SDSS photometry alone, and to identify new WIRRing stars. A more comprehensive survey of stars identified using the methods developed in this paper will lead to a much improved understanding of common envelope evolution.