Masses of Binary stars
An example.  Suppose we measured the period in a
spectroscopic binary and knew the spectral types (and hence the
masses, as we shall see) of the component stars.  The period is 2
years (P = 2 years) and the stars are a G star (1 solar mass) and
a M star (0.5 solar masses).  What is the separation?
MA+MB = 1.5 MSun
1.5 x (2)2 = (a/1 AU)3
6 = (a/1 AU)3
1.8 AU = a