Optical depth in logarithmic scale (colour bar) for different combinations of stellar parameters at orbital phase 0.54. The dependence with the system inclination, i, in the wide range 15–90° (Casares et al. 2005a) is very low at this orbital phase, although it is strong around periastron. The terminal wind velocity is plotted between 600 km s^-1 (wind velocity at the equator in McSwain et al. 2011) and 2000 km s^-1. The expected stellar temperature ranges from 21 500 K McSwain et al. (2011) to 30 000 K. Casares et al. (2005a) estimated the stellar mass to be between 10 and 15 M_⊙. The black circles mark the values adopted in this paper.