Evolutionary tracks of mass donors (dotted lines) and mass gainers (solid lines) of the analysed six binary models in the Hertzsprung-Russell diagram. Pairs of tracks with the same colour belong to the same binary system. The thick grey line marks the zero-age main sequence for the initial mass range of our models (i.e. from 8 to 25 M_⊙). The thick solid drawn parts of the mass gainers’ tracks marks the phase during which the companion is a He star (starting from a core helium mass fraction of Y_c = 0.95 during core helium burning). Corresponding area in HR diagram is coloured light blue and labelled “Be stars”. On the tracks of the mass donors, dots are placed every 10⁵ yr during core helium burning, and a star symbol marks the time at which their core helium mass fraction is Y_c = 0.80. The tracks end during the phase of shell helium burning with a small remaining lifetime of the He stars, except for the System No. 1, which ends at Y_c = 0.7. Area in the HR diagram in which He star models spend most of their lifetime is coloured in pink and labelled “He stars”. Tracks correspond to binary models Nos. 1 to 6 (Table 2) in increasing order of helium-star luminosity (as marked by the star symbols).