  We present the results of a radial velocity survey designed to measure the
fraction of double degenerates among DA white dwarfs. The narrow core of the
H-alpha line was observed twice or more for 46 white dwarfs yielding radial
velocities accurate to a few km/s. This makes our survey the most sensitive to
the detection of double degenerates undertaken to date. We found no new double
degenerates in our sample, though H-alpha emission from distant companions is
seen in two systems. Two stars known to be double degenerates prior to our
observations are included in the analysis. We find a 95% probability that the
fraction of double degenerates among DA white dwarfs lies in the range
[0.017,0.19].
