  We report the analysis of an XMM-Newton observation of the close binary
HD159176 (O7V + O7V). The observed L_X/L_bol ratio reveals an X-ray luminosity
exceeding by a factor ~ 7 the expected value for X-ray emission from single
O-stars, therefore suggesting a wind-wind interaction scenario. EPIC and RGS
spectra are fitted consistently with a two temperature mekal optically thin
thermal plasma model, with temperatures ranging from ~ 2 to 6 10^6 K. At first
sight, these rather low temperatures are consistent with the expectations for a
close binary system where the winds collide well before reaching their terminal
velocities. We also investigate the variability of the X-ray light curve of
HD159176 on various short time scales. No significant variability is found and
we conclude that if hydrodynamical instabilities exist in the wind interaction
region of HD159176, they are not sufficient to produce an observable signature
in the X-ray emission. Hydrodynamic simulations using wind parameters from the
literature reveal some puzzling discrepancies. The most striking one concerns
the predicted X-ray luminosity which is one or more orders of magnitude larger
than the observed one. A significant reduction of the mass loss rate of the
components compared to the values quoted in the literature alleviates the
discrepancy but is not sufficient to fully account for the observed luminosity.
Because hydrodynamical models are best for the adiabatic case whereas the
colliding winds in HD159176 are most likely highly radiative, a totally new
approach has been envisaged... (see paper for complete abstract)
