A B S T R A C TA nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf±Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf±Rayet binary frequency to be 52^14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf±Rayet binaries. Lower ambient metallicity (Z) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf±Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf±Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z. By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC 1 O binaries in the Large Magellanic Cloud all have periods well below 1000 d. The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf±Rayet stars in the Magellanic Clouds, or the formation of Wolf±Rayet stars in general.

A B S T R A C TWe have obtained complete phase coverage of the WC71O binaries WR 42 HD 97152 and WR 79 HD 152270 with high signal-to-noise ratio (S/N), moderate-resolution spectra. Remarkable orbital phase-locked profile variations of the C iii l 5696 line are observed and interpreted as arising from colliding wind effects. Within this scenario, we have modelled the spectra using a purely geometrical model that assumes a cone-shaped wind±wind interaction region which partially wraps around the O star. Such modelling holds the exciting promise of revealing a number of interesting parameters for WR1O binaries, such as the orbital inclination, the streaming velocity of material in the interaction region and the ratio of wind momentum flux. Knowledge of these parameters in turn leads to the possibility of a better understanding of WR star masses, mass-loss rates and wind region characteristics.

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