DISCOVERY OF THE ORBIT OF THE X-RAY PULSAR OAO 1657-415

Timing observations of the 38 s accreting X-ray pulsar OAO 1657-415 made with the BATSE large-area detectors on the Compton Gamma Ray Observatory have revealed a binary orbit with an X-ray eclipse by the stellar companion. Arrival time analysis of 20-60 keV data yielded the following best-fit orbital elements: P(orb) = 10.4436d +/- 0.0038d, a(x) sin i = 106.0 +/- 0.5 It-sec, e = 0.104 +/- 0.005, omega = 93-degrees + 5-degrees, T(pi/2) = JD 2,448,516.49 +/- 0.05 TDB. From the pulsar mass function f(x)(M) = 11.7 +/- 0.2 M. and the measured eclipse half-angle theta(e) = 29.7-degrees +/- 1.3-degrees, we infer that the stellar companion is a supergiant of spectral class BO-B6. If the companion can be identified and its orbital velocity measured, the neutron star mass can be constrained. Both intrinsic spin-up (P/P almost-equal-to -8 x 10(-3) yr-1) and spin-down (P/P almost-equal-to 2 x 10(-3) yr-1) of the pulsar were measured during our observation.