 In general, for any formula_20 or formula_3,

formula_38

In an orbit with eccentricity formula_39, the mass function is given by

formula_40

If the accretor in an X-ray binary has a minimum mass that significantly exceeds the Tolman–Oppenheimer–Volkoff limit (the maximum possible mass for a neutron star), it is expected to be a black hole.This is the case in Cygnus X-1, for example, where the radial velocity of the companion star has been measured.An exoplanet causes its host star to move in a small orbit around the center of mass of the star-planet system.