Timing of the 2008 Outburst of SAX J1808.4-3658 with XMM-Newton: A
  Stable Orbital Period Derivative over Ten Years
  We report on a timing analysis performed on a 62-ks long XMM-Newton
observation of the accreting millisecond pulsar SAX J1808.4-3658 during the
latest X-ray outburst that started on September 21, 2008. By connecting the
time of arrivals of the pulses observed during the XMM observation, we derived
the best-fit orbital solution and a best-fit value of the spin period for the
2008 outburst. Comparing this new set of orbital parameters and, in particular,
the value of the time of ascending-node passage with the orbital parameters
derived for the previous four X-ray outbursts of SAX J1808.4-3658 observed by
the PCA on board RXTE, we find an updated value of the orbital period
derivative, which turns out to be $\dot P_{\rm orb} = (3.89 \pm 0.15) \times
10^{-12}$ s/s. This new value of the orbital period derivative agrees with the
previously reported value, demonstrating that the orbital period derivative in
this source has remained stable over the past ten years. Although this timespan
is not sufficient yet for confirming the secular evolution of the system, we
again propose an explanation of this behavior in terms of a highly
non-conservative mass transfer in this system, where the accreted mass (as
derived from the X-ray luminosity during outbursts) accounts for a mere 1% of
the mass lost by the companion.
