  We report the results of XMM-Newton and BeppoSAX observations of the radio
and X-ray emitting star LS I +61 303, likely associated with the gamma-ray
source 2CG 135+01 and recently detected also at TeV energies. The data include
a long XMM-Newton pointing carried out in January 2005, which provides the
deepest look ever obtained for this object in the 0.3-12 keV range. During this
observation the source flux decreased from a high level of 13E12 erg/cm2/s to
4E12 erg/cm2/s within 2-3 hours.This flux range is the same seen in shorter and
less sensitive observations carried out in the past, but the new data show for
the first time that transitions between the two levels can occur on short time
scales. The flux decrease was accompanied by a significant softening of the
spectrum, which is well described by a power law with photon index changing
from 1.62+/-0.1 to 1.83+/-0.1. A correlation between hardness and intensity is
also found when comparing different short observations spanning almost 10 years
and covering various orbital phases.LS I +61 303 was detected in the 15-70 keV
range with the PDS instrument in one of the BeppoSAX observations, providing
evidence for variability also in the hard X-ray range. The X-ray spectra,
discussed in the context of multiwavelength observations, place some
interesting constraints on the properties and location of the high-energy
emitting region.
