X-ray timing of the 34 millisecond binary pulsar SAX J0635+0533

Modulations detected in the observed spin period of the 34 ms X-ray pulsar SAX J0635+0533 show that the pulsar is in a binary system with a period of 11.2 +/- 0.5 days, a projected orbital semimajor axis a sin i = 83 +/- 11 I t-s, a mass function in the range of 3.4-7.8 M-circle dot, and an orbital e ccentricity of 0.29 +/- 0.09. A combination of observations spanning 2 yr p laces a lower bound on the intrinsic period derivative of the pulsar of 3.8 x 10(-13). The period derivative implies a high torque on the neutron star that would require a very high mass accretion rate, in excess of 6 x 10(-7 ) M-circle dot yr(-1), if produced via accretion. The torque and X-ray lumi nosity are both within the range typical of young, rotation-powered pulsars .