If you go back and watch Trial of a Time Lord, you'll have an answer which was implied *but not for certain* during the episode.
You mean the Valygar(sp)? I don't think that is it. I mean it could be, but that is one and a half incarnations away. I'm not sure I buy that. I'm leaning towards the 9.5 incarnation theory myself. I guess we'll just have to wait and see.