Thanks for your comments. In a two-body problem system, the angular momentum is not constant as stated by the Newton law. The rate of change of momentum (torque) is alternating but due to gravitational waves radiation energy loss, the torque is not symetrical with respect to the perihelion and aphelion line of axis. This explains the mercury perihelion advance. For more details see topic gravitational waves radiation.PDF in
www.gravitomagnetism.com