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Two new Double-lined Spectroscopic Binary White Dwarfs

We present radial velocity observations of four binary white dwarf candidates identified through their over-luminosity. 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 $1.44 \pm 0.06$ and WD 1606+422 is a 20.1 h period system with a mass ratio of $1.33 \pm 0.03$. 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 timescales ranging from minutes to decades. We present an overview of the 14 over-luminous white dwarfs that were identified by Bedard et al., and find the fraction of double- and single-lined systems to be both 31%. However, an additional 31% of these over-luminous 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 over-abundance of single low-mass white dwarfs identified in the SPY survey, and suggest that some of those systems are also likely long period binary systems of more massive white dwarfs.