A friend of mine mentioned the search for "Nemesis" the other day and I completely drew a blank. It seems that there are a bunch of people who think the Sun might have a 'companion' personally I'm not convinced but I do believe that there are probably some ultra cool stars in between us and Proxima Centauri (the closest stellar object we know of) that are yet to have been detected.
A good read can be found at: http://www.space.com/scienceastronomy/solarsystem/nemesis_010320-1.html and http://www.lbl.gov/Science-Articles/Archive/extinctions-nemesis.html - some interesting stuff but I wouldn't believe all that you read!
If Nemesis exists, it may be detected by the planned Pan-STARRS or LSST astronomical surveys, or similar future projects. If Nemesis is a brown dwarf, then the upcoming WISE mission should find it (and it should be quite easy!).
