binary companion

The term binary companion refers to a member-star of a binary star. The term clearly applies to such stars, often meant include stellar remnants and/or brown dwarfs but generally not to a companion that has been established to be an extra-solar planet. The term candidate companion is used for possible such astronomical objects for which evidence is considered inconclusive regarding the existence of some companion and/or whether it qualifies as a star.

Note that the term binary companion is also seen applying to the companion of a planet or minor planet. The term has been used for solar system objects that are co-orbiting an object of similar size.