The orbit of the binary X-ray pulsar 4U 1538-52 from Rossi X-ray Timing Explorer observations

Improved orbital parameters of the high-mass binary X-ray pulsar 4U 1538-52 have been derived from high count rate data obtained from the Rossi X-Ray Timing Explorer. Pulse-timing analysis yields an eccentricity of 0.174 +/- 0.015, a periastron at 64 degrees +/- 9 degrees, and evidence of orbital decay with P-orb/P-orb = (-2.9 +/- 2.1) x 10(-6) yr(-1).