EPIC-pn hardness ratios of the OB stars in Cyg OB2. The solid lines yield the synthetic hardness ratios of absorbed single temperature thermal plasma models for temperatures of kT = 0.4, 0.6, and 0.85 keV (from bottom to top). The neutral column density increases along these lines from 0.5 to 1.5 × 10²² cm^-2 going from left to right. The dashed curve in turn indicates the hardness ratios of the “typical” O-star two-temperature X-ray spectral model determined by Nazé (2009) with an absorption column-density increasing from 0.5 to 2.0 × 10²² cm^-2.