Abstract. Optical photometry for the pulsar PSR 0531+21 has been extended with new observations that strengthen evidence for a previously observed 60 s periodicity. This period is found to be increasing with time at approximately the same rate as the rotational period of the pulsar. The observed period and its time dependence fit a simple free precession model.

The HST High Speed Photometer data and stroboscopic photometry data from the Asiago 1.82m telescope (1994) show a (0.0065±0.0015) magnitude weak modulation peak in Crab’s light-curve at 60 seconds. Hubble data also suggest a phase modulation peak at the same period with the amplitude (0.07 ± 0.02)°. New stroboscopic data from December 1995 Asiago observations suggests the modulation amplitude (0.0045±0.0015) magnitude, consistent with previous results.