Roche lobe radius at periastron versus the companion mass, assuming an orbital period of 2.13 days, for different eccentricities (solid line: e=0; dashed line: e = 0.05; dash-dotted line: e = 0.1). Radii and masses of O-type stars are also overplotted, taken from Martins et al. (2005): diamonds mark O-type giant stars (luminosity class III), while asterisks indicate O-type supergiants (luminosity class I), for different spectral subtypes.