  As neutron star - black hole binaries are expected to be discovered through
future pulsar surveys using upcoming facilities, it is necessary to understand
various observable properties of such systems. In the present work, we study
the advance of the periastron of such binaries under the post-Newtonian
formalism over a wide range of parameters. We find that the first and second
order post-Newtonian effects and the leading order spin-orbit coupling effects
are significant for such binaries but higher order effects can be neglected.
