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We report on a timing analysis performed on a 62-ks long XMM-Newtonobservation of the accreting millisecond pulsar SAX J1808.4-3658 during thelatest X-ray outburst that started on September 21, 2008. By connecting thetime of arrivals of the pulses observed during the XMM observation, we derivedthe best-fit orbital solution and a best-fit value of the spin period for the2008 outburst. Comparing this new set of orbital parameters and, in particular,the value of the time of ascending-node passage with the orbital parametersderived for the previous four X-ray outbursts of SAX J1808.4-3658 observed bythe PCA on board RXTE, we find an updated value of the orbital periodderivative, which turns out to be $\dot P_{\rm orb} = (3.89 \pm 0.15) \times10^{-12}$ s/s. This new value of the orbital period derivative agrees with thepreviously reported value, demonstrating that the orbital period derivative inthis source has remained stable over the past ten years. Although this timespanis not sufficient yet for confirming the secular evolution of the system, weagain propose an explanation of this behavior in terms of a highlynon-conservative mass transfer in this system, where the accreted mass (asderived from the X-ray luminosity during outbursts) accounts for a mere 1% ofthe mass lost by the companion.

Timing of the 2008 Outburst of SAX J1808.4-3658 with XMM-Newton: A Stable Orbital Period Derivative over Ten Years