http://www.nineplanets.org/hypo.html#nemesis
[snip] This hypothetical “death companion” of the Sun was suggested in 1985 by Daniel P. Whitmire and John J. Matese, Univ of Southern Louisiana. It has even received a name: Nemesis. One awkward fact of the Nemesis hypothesis is that there is no evidence whatever of a companion star of the Sun. It need not be very bright or very massive, a star much smaller and dimmer than the Sun would suffice, even a brown or a black dwarf (a planet-like body insufficiently massive to start “burning hydrogen” like a star). It is possible that this star already exists in one of the catalogues of dim stars without anyone having noted something peculiar, namely the enormous apparent motion of that star against the background of more distant stars (i.e. its parallax). If it should be found, few will doubt that it is the primary cause of periodic mass extinctions on Earth... However, since the examination of the entire sky in the far IR by IRAS with no “Nemesis” found, the existence of “Nemesis” is not very likely. [end]
http://www.freerepublic.com/focus/news/1962278/posts?page=46#46
http://muller.lbl.gov/teaching/Physics10/Nemesis%20book/