Evolution of the stripped donor star of HIP 15429 in the HRD. The model track (black line) was computed for a binary system with initial masses of 5.5 M_⊙ and 4.95 M_⊙ and an orbital period of 8 days. The black square marker indicates the present-day values of the stripped star, estimated from spectroscopy (effective temperature) and SED fitting (luminosity). Coloured sections mark critical evolutionary episodes. Orange markers are spaced in regular time intervals of 5 × 10⁴ years starting at the time of first Roche-lobe overflow.