So, they have spotted the "coolest" known brown dwarf. (It must have some very big sunglasses on). http://arxiv.org/PS_cache/arxiv/pdf/110 ... 5411v1.pdf
It is the companion to a white dwarf. It is separated from its companion by 2500 AU. Companion stars are generally coeval. White dwarfs are generally pretty old/advanced in their evolution. So that would mean that the Brown dwarf is also quite old. Brown dwarfs by definition do not have enough mass to have nuclear fusion going on.
My question is this, what is keeping it so warm after such a long time? Its temperature is 300k.