The fraction of double degenerates among DA white dwarfs

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].