Expansion-induced contribution to the precession of binary orbits

We point out the existence of new effects of global spacetime expansion on local binary systems. In addition to a possible change of orbital size, the re is a contribution to the precession of elliptic orbits, to be added to t he well known general relativistic effect in static spacetimes, and the ecc entricity can change. Our model calculations are done using geodesics in a McVittie metric, representing a localized system in an asymptotically Rober tson-Walker spacetime; we give a few numerical estimates for that case, and briefly comment on ways in which the model should be improved.