New ephemeris of the ADC source 2A 1822-371: a stable orbital-period
  derivative over 30 years
  We report on a timing of the eclipse arrival times of the low mass X-ray
binary and X-ray pulsar 2A 1822-371 performed using all available observations
of the Proportional Counter Array on board the Rossi X-ray Timing Explorer,
XMM-Newton pn, and Chandra. These observations span the years from 1996 to
2008. Combining these eclipse arrival time measurements with those already
available covering the period from 1977 to 1996, we obtain an orbital solution
valid for more than thirty years. The time delays calculated with respect to a
constant orbital period model show a clear parabolic trend, implying that the
orbital period in this source constantly increases with time at a rate $\dot
P_orb = 1.50(7) \times 10^{-10}$ s/s. This is 3 orders of magnitude larger than
what is expected from conservative mass transfer driven by magnetic braking and
gravitational radiation. From the conservation of the angular momentum of the
system we find that to explain the high and positive value of the orbital
period derivative the mass transfer rate must not be less than 3 times the
Eddington limit for a neutron star, suggesting that the mass transfer has to be
partially non-conservative. With the hypothesis that the neutron star accretes
at the Eddington limit we find a consistent solution in which at least 70% of
the transferred mass has to be expelled from the system.
