  We report on the results obtained for the millisecond transient X-ray pulsar
XTE J1807-294 in a 40 min orbital period system, based on an XMM-Newton ToO
observation carried out during March 2003. The source was found at a luminosity
level of about 2x10^36 erg/s in the 0.5-10 keV range (assuming a distance of 8
kpc). We confirm the presence of the 5.25 ms pulsations (after accounting for
the orbital modulation) and find a pulsed fraction of 5.8% in the 0.3-10 keV
band. The pulse shape in nearly sinusoidal. The spectral continuum of the
source is well fitted by an absorbed Comptonization model plus a soft
component. No emission or absorption lines have been detected in the 0.5-10 keV
range with upper limits of 10-40 eV. The reported analysis represent the first
detailed study of this source, the fourth belonging to the ultra-compact binary
system class hosting an accreting neutron star.
