  We summarize the results of a multiwavelength observing campaign on the
massive eclipsing binary CPD-41deg7742, another remarkable object in the young
open cluster NGC 6231. Our campaign relies on high resolution echelle
spectroscopy, narrow-band optical photometry, and XMM-Newton X-ray
observations. Combined with the spectroscopic analysis, the light curve
analysis provides a direct measurement of the masses and sizes of the system
components. However, the most outstanding results come from the XMM-Newton
observations. Our 180 ks campaign towards NGC 6231, and CPD-41deg7742, provides
an unprecedented phase coverage of such a close early-type binary. The EPIC-MOS
light curves almost fully cover the 2.4 day period of the system and the
brightness of the object is sufficient to yield a time resolution as tight as 1
ks. The X-ray flux presents clear variations along the orbit, that we interpret
as the signature of an unusual wind interaction. We indeed expect that, in this
O+B system, the dominant primary wind crashes into the secondary surface,
leading to a wind-photosphere interaction. As a strong support to our
interpretation, we provide a geometrical model that associates an extra X-ray
emission to the secondary inner surface. Though quite simple, the present model
matches the main features of the X-ray light curve.
