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We report on a timing of the eclipse arrival times of the low mass X-raybinary and X-ray pulsar 2A 1822-371 performed using all available observationsof the Proportional Counter Array on board the Rossi X-ray Timing Explorer,XMM-Newton pn, and Chandra. These observations span the years from 1996 to2008. Combining these eclipse arrival time measurements with those alreadyavailable covering the period from 1977 to 1996, we obtain an orbital solutionvalid for more than thirty years. The time delays calculated with respect to aconstant orbital period model show a clear parabolic trend, implying that theorbital period in this source constantly increases with time at a rate $\dotP_orb = 1.50(7) \times 10^{-10}$ s/s. This is 3 orders of magnitude larger thanwhat is expected from conservative mass transfer driven by magnetic braking andgravitational radiation. From the conservation of the angular momentum of thesystem we find that to explain the high and positive value of the orbitalperiod derivative the mass transfer rate must not be less than 3 times theEddington limit for a neutron star, suggesting that the mass transfer has to bepartially non-conservative. With the hypothesis that the neutron star accretesat the Eddington limit we find a consistent solution in which at least 70% ofthe transferred mass has to be expelled from the system.

New ephemeris of the ADC source 2A 1822-371: a stable orbital-period derivative over 30 years