  We present a combined analysis of XMM-Newton, Chandra and Rosat observations
of the isolated neutron star RX J0720.4-3125, spanning a total period of \sim 7
years. We develop a maximum likelihood periodogramme based on \Delta C
statistic and maximum likelihood method, which are appropriate for sparse event
lists. As an "a posteriori" check, we have folded a further BeppoSAX dataset
with the period predicted at the time of that observation, finding that the
phase is consistent. The value of the spin down rate, here measured for the
first time, is \approx 10^{-14} s/s and can not be explained in terms of
propeller or torque from a fossil disk. When interpreted in terms of dipolar
losses, it gives a magnetic field of B \approx 10^{13} G, making also
implausible that the source is accreting from the underdense surroundings. We
discuss the implications of this measure for the different mechanisms that have
been suggested to explain the X-ray emission. We conclude that the observed
properties are more compatible with a scenario in which the source is \approx
10^6 yrs old, and its magnetic field has not changed substantially over the
lifetime.
