Location of the single WO stars in the Hertzsprung-Russell diagram. Also indicated are the WC stars analyzed by Sander et al. (2012) and evolutionary tracks for solar metallicity and an initial rotational veloctity of 40% critical from Ekström et al. (2012). The helium zero-age main sequence (He-ZAMS) from BEC models (which include envelope inflation) is indicated for solar metallicity (solid line) and SMC metallicity (dashed line).