Orbital variability of the PSR J2051-0827 binary system

We have carried out high-precision timing measurements of the binary millis econd pulsar PSR J2051-0827 with the Effelsberg 100-m radio telescope of th e Max-Planck-Institut fur Radioastronomie and with the Lovell 76-m radio te lescope at Jodrell Bank. The 6.5-yrs radio timing measurements have reveale d a significant secular variation of the projected semi-major axis of the p ulsar at a rate of (x)over dot equivalent to d(a(1) sin i)/dt = (-0.23 +/-0 .03) x 10(-12), which is probably caused by the Newtonian spin-orbit coupli ng in this binary system leading to a precession of the orbital plane. The required misalignment of the spin and orbital angular momenta of the compan ion are evidence for an asymmetric supernova explosion. We have also confir med that the orbital period is currently decreasing at a rate of (P)over do t(b) = (-15.5 +/-0.8) x 10(-12) s s(-1) and have measured second and third orbital period derivatives d(2)P(b)/dt(2) = (+2.1 +/-0.3) x 10(-20) s(-1) a nd d(3)P(b)/dt(3) = (3.6 +/-0.6) x 10(-28) s(-2), which indicate a quasi-cy clic orbital period variation similar to those found in another eclipsing p ulsar system, PSR B1957+20. The observed variation of the orbital parameter s constrains the maximal value of the companion radius to R-c max similar t o 0.06 R-circle dot and implies that the companion is underfilling its Roch e lobe by 50%. The derived variation in the quadrupole moment of the compan ion is probably caused by tidal dissipation similar to the mechanism propos ed for PSR B1957+20. We conclude that the companion is at least partially n on-degenerate, convective and magnetically active.