Post-Newtonian effects in the motion of two bodies of comparable masses

The motion of binary systems consisting of self-gravitating bodies of comparable masses is examined within the framework of an extended version of the parametrized post-Newtonian formalism. It is shown that the dynamic characteristics of moving bodies significantly depend on a new parameter $\tau$. A possible method of evaluating this parameter in treating the observation data obtained from the study of the pulsar system PSR 1913+16 is discussed.