Comparison of synthetic spectra in the blue optical range obtained for models with different stellar temperatures and surface gravities. The mean optical spectrum of M33 X-7 (Orosz et al. 2007) is shown as a black solid line. The deep cores in some of the Balmer lines are likely due to an over-subtraction of nebular emission (see supplementary information in Orosz et al. 2007). In contrast to the other He II lines, He II 4686Å appears as a weak emission in the observation. This line might be filled up with nonstellar emission, e.g. from the accretion disk.