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P. Kaaret (Harvard-Smithsonian CfA), G. Cusumano, B. Sacco (IFCAI/CNR, Palermo)
Modulations detected in the observed spin period of the 34~ms x-ray pulsar SAX J0635+0533 have allowed us to measure the orbital parameters of the system. Combination of observations spanning two years place a lower bound on the intrinsic period derivative of the pulsar of 3.8 \times 10-13. The period derivative implies a high torque on the neutron star which would require a very high mass accretion rate, in excess of 6 \times 10-7 \rm \, M\odot \, yr-1, if produced via accretion. The torque and x-ray luminosity are both within the range typical of young, rotation-powered pulsars. We suggest that SAX~J0635+0533 is a very young, less than 1400 year old, rotation-powered pulsar in a binary system.