Brown Dwarf Companions to White Dwarfs

Brown dwarf companions to white dwarfs are rare, but recent infra-red surveys are slowly revealing examples. We present new observations of the post-common envelope binary WD0137−349, which reveals the effects of irradiation on the ≈ 0.05M⊙ secondary, and new observations of GD 1400 which show that it too is a close, post-common envelope system. We also present the latest results in a near-infrared photometric search for unresolved ultra-cool companions and to white dwarfs with UKIDSS. Twenty five DA white dwarfs were identified as having photometric excesses indicative of a low mass companion, with 8-10 of these having a predicted mass in the range associated with brown dwarfs. The results of this survey show that the unresolved (< 2”) brown dwarf companion fraction to DA white dwarfs is 0.3 ≤ fWD+BD ≤ 1.3%