The mass function is a numerical relation between the masses of the two components of a spectroscopic binary that provides information on the relative masses of the two stars when the spectral lines of only one component can be seen. If M_p is the mass of primary (whose spectrum is known), M_s is the mass of secondary, and i is the inclination of the orbit, the mass function is given by (M_s³ sin³i) / (M_p + M_s)².