  We analyse two ASCA observations of the highly eccentric O9III + B1III binary
Iota Orionis obtained at periastron and apastron. Based on the assumption of a
strong colliding winds shock between the stellar components, we expected to see
significant variation in the X-ray emission between these phases. The
observations proved otherwise: the X-ray luminosities and spectral
distributions were remarkably similar. The only noteworthy feature was the hint
of a proximity effect during periastron passage, supported also in the optical.
We discuss the accuracy of our results, and also analyse archival ROSAT
observations. We investigate why we do not see a clear colliding winds
signature. A simple model shows that the wind attenuation to the expected
position of the shock apex is negligible throughout the orbit, which poses the
puzzling question of why the expected 1/D variation (ie. a factor of 7.5) in
the intrinsic luminosity is not seen in the data. Two scenarios are proposed:
either the colliding winds emission is unexpectedly weak such that intrinsic
shocks in the winds dominate the emission, or, alternatively, that the emission
observed is colliding winds emission but in a more complex form than we would
naively expect. Complex hydrodynamical models are then analyzed. Despite
strongly phase-variable emission from the models, both were consistent with the
observations. We find that if the mass-loss rates of the stars are low then
intrinsic wind shocks could dominate the emission. However, when we assume
higher mass-loss rates of the stars, we find that the observed emission could
also be consistent with a purely colliding winds origin. To distinguish between
the different models X-ray observations with improved phase coverage will be
necessary.
