The Binary Fraction of Low Mass White Dwarfs
  We describe spectroscopic observations of 21 low-mass (<0.45 M_sun) white dwarfs (WDs) from the Palomar-Green Survey obtained over four years. We use both radial velocities and infrared photometry to identify binary systems, and find that the fraction of single, low-mass WDs is <30%. We discuss the potential formation channels for these single stars including binary mergers of lower-mass objects. However, binary mergers are not likely to explain the observed number of single low-mass WDs. Thus additional formation channels, such as enhanced mass loss due to winds or interactions with substellar companions, are likely. 
Justin M. Brown (1), Mukremin Kilic (2), Warren R. Brown (2), and   Scott J. Kenyon (3) ((1) Franklin and Marshall College (...
astro-ph.GA astro-ph.SR