Recent Timing Studies on RXTE Observations of 4U 1538-52
  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. 
A. Baykal (1), S.C. Inam (2), E. Beklen (1,3) ((1) Physics Department,   Middle East Technical University, Ankara, Turkey (2...