Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque   in IGR J00291+5934
  We present here a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of December 2004. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we find a clear parabolic trend of the pulse phase delays showing that the pulsar is spinning up as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar. 
L. Burderi, T. Di Salvo, A. Riggio, M.T. Menna, G. Lavagetto, A.   Papitto, R. Iaria, N. R. Robba, L. Stella