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 Hα line was observed twice or more for 46 white dwarfs yielding radial velocities accurate to a few km s−1. 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 Hα 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 per cent probability that the fraction of double degenerates among DA white dwarfs lies in the range [0.017, 0.19].