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.
@article{arxiv.1101.5169,
title = {The Binary Fraction of Low Mass White Dwarfs},
author = {Justin M. Brown and Mukremin Kilic and Warren R. Brown and Scott J. Kenyon},
journal= {arXiv preprint arXiv:1101.5169},
year = {2015}
}