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 J2051-0827 with the European Pulsar Timing Array network of
telescopes and the Parkes radio telescope, spanning over 13 years. The
increased data span allows significant measurements of the orbital
eccentricity, e = (6.2 {\pm} 1.3) {\times} 10^{-5} and composite proper motion,
{\mu}_t = 7.3 {\pm} 0.4 mas/yr. Our timing observations have revealed secular
variations of the projected semi-major axis of the pulsar orbit which are much
more extreme than those previously published; and of the orbital period of the
system. Investigation of the physical mechanisms producing such variations
confirm that the variations of the semi-major 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.
