Magellanic Cloud WC/WO Wolf-Rayet stars - I. Binary frequency and Roche lobe overflow formation

A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf-Rayet s tars is studied spectroscopically and photometrically to determine its bina ry frequency. Theory predicts the Roche lobe overflow produced Wolf-Rayet b inary 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 ove rflow Wolf-Rayet binaries. Lower ambient metallicity (Z) leads to lower opa city, preventing all but the most massive (hence luminous) single stars fro m reaching the Wolf-Rayet stage. However, theory predicts that Roche lobe o verflow even in binaries of modest mass will lead to Wolf-Rayet stars in bi naries with periods below approximately 200 d, for initial periods below ap proximately 1000 d, independent of Z, By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths an d 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 bin ary status actually turn out to be binary, current theory and observation w ould agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC + O binaries in the Large Magellanic Cloud all have periods well below 1000 d, The large majority of WC/WO star s in such environments apparently can form without the aid of a binary comp anion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf-Rayet stars in the Magellanic Cl ouds, or the formation of Wolf-Rayet stars in general.