The Schwarzschild metric allows to calculate the precession of an elliptic orbit of a particle around a large central mass, provided the mass of the particle is much smaller than the central mass M. This condition is not met in the case of binary stars m1 and m2 revolving around each other. There is mention of a possibility of still using the Schwarzschild metric, but then one must take the sum of the masses of the two stars for the central mass, i.e. M=m1+m2. I can imagine this works, but don't know how. Could anyone supply a reference on this subject. Thanks!