Determining orbits and masses of binary stars

Much of the information we have about X-ray binaries has come from the study of X-ray pulsars. X-ray pulsars allow the determination of the orbital parameters of the binary system by the Doppler delays of the pulse arrival times. If the Doppler velocity curve of the primary is also available, the mass of the secondary star can be determined. This is how we know that the secondary must be a neutron star or black hole - all of the masses which have been determined are consistent with ~1.4 solar mass neutron stars or with higher mass black holes.