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The sign and magnitude of the orbital period derivative is perhaps the most critical diagnostic of the evolution of low mass X-ray binaries. One of the very few with (possibly) a measured orbital period derivative is the eclipsing X-ray binary EXO0748-676, whose sharp edged eclipse transitions (possibly) provide a good orbital fiducual marker. We have combined ROSAT observations of EXO0748-676 with EXOSAT (Parmar et al., ApJ 366:253, 1991) and Ginga (Asai et al., PASJ 44:633, 1992) observations to extend the ephemeris determination. A standard analysis of the 7 years of data shows that the eclipse timings for EXO0748-676 are equally well fitted by an ephemeris with a constant second period derivative or a 12 year sinusoidal variation in the orbital period.
Standard analysis assumes that measured eclipse times consist of the deterministic time of the eclipse plus measurement error. If there is any phase jitter in the true time of the eclipse, then the assumption that the timing residuals plotted as an O-C diagram are independent is invalid. The intrinsic jitter will accumulate in the O-C residuals, and the O-C plot will show a random walk behavior. Using statistical tests recently brought to our attention by F.~Lombard (Padua Time Series Analysis Conference, September 1993), we show that the behavior of EXO0748-676 is statistically consistent with the eclipse timing residuals being due to stochastic behavior in the X-ray source rather than a secular change in the orbital period. We show that similar behavior is observed in cataclysmic variables and variable stars, and suggest physical mechanisms.
