Abstract
The high mass X-ray binary pulsar 4U 1538-52 was observed between July 31 and
August 7, 2003. Using these observations, we determined new orbital epochs for
both circular and elliptical orbit models. The orbital epochs for both orbit
solutions agreed with each other and yielded an orbital period derivative
$\dot{P} / P = (0.4 \pm 1.8) \times 10^{-6}$ yr$^{-1}$. This value is
consistent with the earlier measurement of $\dot{P} / P = (2.9 \pm 2.1) \times
10^{-6}$ yr$^{-1}$ at the $1 \sigma$ level and gives only an upper limit to the
orbital period decay. Our determination of the pulse frequency showed that the
source spun up at an average rate of $2.76 \times 10^{-14}$ Hz sec$^{-1}$
between 1991 and 2003.