A Revised Orbital Period for M33 X-7

We analyzed the X-ray light curve of M33 X-7 using observations made by the Einstein Observatory, ROSAT, and ASCA satellites. Based on the high and low states characteristic of an eclipsing X-ray binary, we find evidence for a constant orbital period of P_orb = 3.4531 days and a trapezoidal light curve with a fall time of 0.6 days, a total eclipse lasting 0.3 days, and a rise time of 0.2 days. The 99% confidence interval for the rate of orbital period change is -5 x 10(-6) <=\ P_orb/P_orb <=\ 9 x 10(-6) yr(-1) . Because these extremes represent rates of period change that are larger than any so far observed in X-ray binaries, further observations are necessary to obtain stricter constraints on the rate of orbital period change in M33 X-7.