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Results: The two Wolf-Rayet stars WR20a (WN6ha+WN6ha) and WR20b (WN6ha) wereanalyzed in detail. They are both very luminous and display very hard spectra,but WR20b does not seem to vary. On the contrary, WR20a, a known eclipsing,colliding-wind binary, brightens in the X-ray domain during the eclipses, i.e.when the collision is seen face-on. This can be explained by the properties ofthe wind-wind collision zone, whose high density leads to a large absorbingcolumn (2 10^24 cm^-2).All twelve O-type stars previously classifiedspectroscopically, two eclipsing binaries previously identified and nine newlyidentified O-type star candidates are detected in the high energy domain; tenof them could be analyzed spectroscopically. Four are overluminous, but theothers present typical L_X/L_BOL ratios, suggesting that several O-type objectsare actually binaries. Variability at the ~2sigma level was detected for amajority of the sources, of unknown origin for the putatively single objects.Faint, soft, diffuse emission pervades the entire field-of-view but no clearstructure can be identified, even at the position of a blister proposed to beat the origin of the TeV source HESS J1023-575. Finally, the X-ray propertiesof PMS objects were also investigated, in particular the brightest flaringones. They provided an additional argument in favor of a large distance (~8kpc)for the cluster.

Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund2