Barnard's Star
If it does exist, the exact nature of Nemesis is uncertain. Muller suggests that the most likely object is a red dwarf with an apparent magnitude between 7 and 12, while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf. If a red dwarf, it would exist in star catalogs, but it would only be confirmed by measuring its parallax; due to orbiting the Sun it would have a low proper motion and would escape detection by older proper motion surveys that have found stars like the 9th-magnitude Barnard's Star. (The proper motion of Barnard's Star was detected in 1916.) Muller expects Nemesis to be discovered by the time parallax surveys reach the 10th magnitude.