  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.
