Second Decrease in the Period of the Vela Pulsar

THE first discontinuity in period of the Vela Pulsar (PSR 0833-45) occurred between February 24 and March 3, 1969 (refs. 1 and 2). The period decreased by 208 ns and the rate of change of period increased by 1 × 10^-15 s s^-1. A similar discontinuity occurred between August 21 and September 4, 1971. A preliminary analysis of the data shows that the period decreased by 179 ns and that the rate of change of period increased. A definitive analysis of the discontinuity will have to await more data because of the normally irregular behaviour of the period³.