Ja n 20 05 Geodetic Precession in PSR J 1141 – 6545

We present observations that show dramatic evolution of the mean pulse profile of the relativistic binary pulsar J1141–6545 over a period of five years. This is consistent with precession of the pulsar spin axis due to relativistic spin-orbit coupling. Observations made between 1999 and 2004 with a number of instruments at the Parkes radio telescope demonstrate a steady, secular evolution of the mean total intensity profile, which increases in width by more than 50 percent during the five year period. Analysis of the changing position angle of the linearly polarised component of the mean profile suggests that our line of sight is shifting closer to the core of the emission cone. We find that the slope of the position angle swing across the centre of the pulse steepens with time and use a simplified version of the rotating vector model to constrain the magnitude and direction of the change in our line of sight angle relative to the pulsar magnetic axis. The fact that we appear to be moving deeper into the emission cone is consistent with the non-detection of this pulsar in previous surveys.