Sections of the observed optical spectrum around H I Balmer lines and their respective He II blends compared with three synthetic spectra with T_eff = 45 000 K and different H and He abundances (given in mass fraction, the labels’ colors refer to the respective spectra). A synthetic spectrum calculated with the photospheric parameters of Krtička et al. (2019) is shown in the line centers with a dashed, violet line. Prominent stellar and interstellar (subscript is) lines are marked.