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The orbital period evolution of X-ray binaries provides fundamental clues tounderstanding mechanisms of angular momentum loss from these systems. Wepresent an X-ray eclipse timing analysis of the transient low mass X-ray binaryAX J1745.6-2901. This system shows full eclipses and thus is one of the fewobjects for which accurate orbital evolution studies using this method can becarried out. We report on XMM-Newton and ASCA observations covering 30 completeX-ray eclipses spanning an interval of more than 20 years. We improve thedetermination of the orbital period to a relative precision of$2\times10^{-8}$, two orders of magnitudes better than previous estimates. Wedetermine, for the first time, a highly significant rate of decrease of theorbital period $\dot{P}_{orb}=-4.03\pm0.32\times10^{-11}$~s/s. This is at leastone order of magnitude larger than expected from conservative mass transfer andangular momentum losses due to gravitational waves and magnetic breaking, andmight result from non-conservative mass transfer. Imprinted on the long termevolution of the orbit, we observe highly significant eclipse leads-delays of~10-20 s, characterised by a clear state dependence in which, on average,eclipses occur earlier during the hard state.

The puzzling orbital period evolution of the low mass X-ray binary AX J1745.6-2901