Radial velocity measurements of white dwarfs

We present 594 radial velocity measurements for 71 white dwarfs obtained du ring our search for binary white dwarfs and not reported elsewhere. We iden tify three excellent candidate binaries, which require further observations to confirm our preliminary estimates for their orbital periods, and one ot her good candidate. We investigate whether our data support the existence o f a population of single, low-mass (less than or similar to0.5 M-circle dot ) white dwarfs (LMWDs). These stars are difficult to explain using standard models of stellar evolution. We find that a model with a mixed single/bina ry population is at least similar to 20 times more likely to explain our da ta than a pure binary population. This result depends on assumed period dis tributions for binary LMWDs, assumed companion masses and several other fac tors. Therefore, the evidence in favour of the existence of a population of single LMWDs is not sufficient, in our opinion, to firmly establish the ex istence of such a population, but does suggest that extended observations o f LMWDs to obtain a more convincing result would be worthwhile.