Two new double-lined spectroscopic binary white dwarfs

We present radial velocity observations of four binary white dwarf candidates identified through their overluminosity. We identify two new double-lined spectroscopic binary systems, WD 0311-649 and WD 1606+422, and constrain their orbital parameters. WD 0311-649 is a 17.7 h period system with a mass ratio of 1.44 +/- 0.06 and WD 1606+422 is a 20.1 h period system with a mass ratio of 1.33 +/- 0.03. An additional object, WD 1447-190, is a 43 h period single-lined white dwarf binary, whereas WD 1418-088 does not show any significant velocity variations over time-scales ranging from minutes to decades. We present an overview of the 14 overluminous white dwarfs that were identified by Bedard et al., and find the fraction of double- and single-lined systems to be both 31 per cent. However, an additional 31 per cent of these overluminous white dwarfs do not show any significant radial velocity variations. We demonstrate that these must be in long-period binaries that may be resolved by Gaia astrometry. We also discuss the overabundance of single low-mass white dwarfs identified in the SPY survey, and suggest that some of those systems arc also likely long-period binary systems of more massive white dwarfs.