Evidence for gravitational quadrupole moment variations in the companion of PSR J2051-0827

We have conducted radio timing observations of the eclipsing millisecond binary pulsar PSR J2051-0827 with the European Pulsar Timing Array network of telescopes and the Parkes radio telescope, spanning over 13 yr. The increased data span allows significant measurements of the orbital eccentricity, e = (6.2 +/- 1.3) x 10(-5), and composite proper motion, mu(t) = 7.3 +/- 0.4 mas yr(-1). Our timing observations have revealed secular variations of the projected semimajor axis of the pulsar orbit which are much more extreme than those previously published and of the orbital period of the system. Investigations of the physical mechanisms producing such variations confirm that the variations of the semimajor axis are most probably caused by classical spin-orbit coupling in the binary system, while the variations in orbital period are most likely caused by tidal dissipation leading to changes in the gravitational quadrupole moment of the companion.