Eclipse timings of the low-mass X-ray binary EXO 0748-676 .2. Detection of an apparent orbital period change and of orbital period noise

The rare eclipsing low-mass X-ray binary EXO 0748-676 has now been monitored accurately for over a decade in the hope of detecting orbital period evolution. We report observations of 10 complete eclipses at resolution less than 0.1 s with the Rossi X-ray Timing Explorer during 1996 May and 1996 August. We have timed the eclipses with an accuracy limited by photon statistics to 0.2-0.5 s. When combined with historical measurements, the eclipse timings imply an unexpectedly large orbital period derivative, (P) over dot(orb) = 3.3 x 10(-11), and a rapid timescale for orbital evolution, tau(orb) = 1.3 x 10(7) yr. The observations also require considerable intrinsic jitter in the mideclipse timings; a maximum likelihood estimate is 0.15 s per orbital cycle. We consider various models and scenarios that might account for these unexpected results.