 The true mass formula_4 depends on the orbital inclination.The inclination is typically not known, but to some extent it can be determined from observed eclipses, be constrained from the non-observation of eclipses, or be modelled using ellipsoidal variations (the non-spherical shape of a star in binary system leads to variations in brightness over the course of an orbit that depend on the system's inclination).In the case of formula_30 (for example, when the unseen object is an exoplanet), the mass function simplifies to

formula_31

In the other extreme, when formula_32 (for example, when the unseen object is a high-mass black hole), the mass function becomes

formula_33

and since formula_34 for formula_35, the mass function gives a lower limit on the mass of the unseen object 2.