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We performed a timing analysis of the 2003 outburst of the accreting X-raymillisecond pulsar XTE J1807-294 observed by RXTE. Using recently refinedorbital parameters we report for the first time a precise estimate of the spinfrequency and of the spin frequency derivative. The phase delays of the pulseprofile show a strong erratic behavior superposed to what appears as a globalspin-up trend. The erratic behavior of the pulse phases is strongly related torapid variations of the light curve, making it very difficult to fit thesephase delays with a simple law. As in previous cases, we have thereforeanalyzed separately the phase delays of the first harmonic and of the secondharmonic of the spin frequency, finding that the phases of the second harmonicare far less affected by the erratic behavior. In the hypothesis that thesecond harmonic pulse phase delays are a good tracer of the spin frequencyevolution we give for the first time a estimation of the spin frequencyderivative in this source. The source shows a clear spin-up of $\dot \nu =2.5(7) \times 10^{-14}$ Hz sec$^{-1}$ (1 $\sigma$ confidence level). Thelargest source of uncertainty in the value of the spin-up rate is given by theuncertainties on the source position in the sky. We discuss this systematics onthe spin frequency and its derivative.

Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294