Brown dwarf
Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars will burn their lithium within a little over 100 million years (my), whereas brown dwarfs  will not. Confusingly, brown dwarves have temperatures and luminosities similar to some true stars. In other words, the detection of lithium in the atmosphere of an object means that, if it is older than 100 my, it is a brown dwarf.