| Summary: | Phase-dependent line-profile variations of Wolf-Rayet + O-star binaries provide an opportunity to probe the physical and ionization structure of WR winds and the shock zone between the two stars. A model is developed to calculate the phase-dependent variation in the absorption by the wind, which takes into account the structure of the shock zone, the cavity caused by the O-star wind, and the changing ionization balance of the wind. Observations of the line-profile variations of 2 Vel and HD 152270 are discussed in detail and then compared with the model. The variations of the weaker absorption lines, particularly the FeIV pseudo continuum in 2 Vel are shown to be quantitatively consistent with the expectations of selective absorption. The emission lines on the other hand are shown to vary by more than can be explained by selective absorption alone, and in a way which is inconsistent with selective absorption. These variations are broadly consistent with the scenario of excess emission from the shock zone, fueled by the colliding winds. In both stars the ratios of the emission line strengths relative to single WCL stars suggest unusually strong lines which provides additional support to the idea of excess emission. The level of the excess emission in the lines is probably very large, approaching the intensity of the O star in the lines. The fit to the FeIV pseudo-continuum lines leads to an iron abundance which is approximately 1 - 2 times solar in both 2 Vel and HD 152270. By modelling the region around the N IV 1718 line it is possible to reduce the upper limit of the nitrogen abundance in WC stars by about 2 orders of magnitude, giving C/N > 1000 and > 2000, in 2 Vel and HD 152270 respectively.
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