Geodetic Precession in PSR J1141−6545

We present observations that show dramatic evolution of the mean pulseprofile of the relativistic binary pulsar J1141--6545 over a period of fiveyears. This is consistent with precession of the pulsar spin axis due torelativistic spin-orbit coupling. Observations made between 1999 and 2004 witha number of instruments at the Parkes radio telescope demonstrate a steady,secular evolution of the mean total intensity profile, which increases in widthby more than 50 percent during the five year period. Analysis of the changingposition angle of the linearly polarised component of the mean profile suggeststhat our line of sight is shifting closer to the core of the emission cone. Wefind that the slope of the position angle swing across the centre of the pulsesteepens with time and use a simplified version of the rotating vector model toconstrain the magnitude and direction of the change in our line of sight anglerelative to the pulsar magnetic axis. The fact that we appear to be movingdeeper into the emission cone is consistent with the non-detection of thispulsar in previous surveys.Comment: 22 pages, 7 figures, accepted for publication in Ap