  The short-period (1.64 d) near-contact eclipsing WN6+O9 binary system CQ Cep
provides an ideal laboratory for testing the predictions of X-ray colliding
wind shock theory at close separation where the winds may not have reached
terminal speeds before colliding. We present results of a Chandra X-ray
observation of CQ Cep spanning ~1 day during which a simultaneous Chandra
optical light curve was acquired. Our primary objective was to compare the
observed X-ray properties with colliding wind shock theory, which predicts that
the hottest shock plasma (T > 20 MK) will form on or near the line-of-centers
between the stars. The X-ray spectrum is strikingly similar to apparently
single WN6 stars such as WR 134 and spectral lines reveal plasma over a broad
range of temperatures T ~ 4 - 40 MK. A deep optical eclipse was seen as the O
star passed in front of the Wolf-Rayet star and we determine an orbital period
P = 1.6412400 d. Somewhat surprisingly, no significant X-ray variability was
detected. This implies that the hottest X-ray plasma is not confined to the
region between the stars, at odds with the colliding wind picture and
suggesting that other X-ray production mechanisms may be at work. Hydrodynamic
simulations that account for such effects as radiative cooling and orbital
motion will be needed to determine if the new Chandra results can be reconciled
with the colliding wind picture.
