Hertzsprung-Russell diagram showing the evolution of a star that loses its hydrogen-rich envelope through Roche-lobe overflow in a binary (our reference model, metallicity Z = 0.0166). The color of the track shows the central helium mass fraction, highlighting the long-lasting phases of hydrogen and helium core burning with color change. The thicker, black background line indicates the Roche-lobe overflow. Letters denote different evolutionary stages described in the text. At point F the central helium mass fraction of the stripped star decreases and passes X_He,c = 0.5 – the point when we model the spectrum (see Sect. 3.3 and Figs. 4 and 6). The gray track is that of a corresponding 12 M_⊙ single star. The blue background shading shows the fraction of emitted flux that is hydrogen ionizing, assuming blackbody radiation. The gray straight lines indicate lines of constant radius.