Left: (a1) Evolutionary tracks of four different scenarios of high-mass stars on the HR diagram. A1 and A4 are single-star models (M = 4.5 and M = 3.0 M_⊙), and A2 and A3 are mass-accreting and mass-donating models (M = 3.0 → 4.5 M_⊙ and M = 4.5 → 3.0 M_⊙). (a2) Asymptotic period spacing, ΔP, as a function of the central hydrogen mass fraction, X_c. (a3) Profiles of hydrogen mass fraction, X, for selected models, labelled by different symbols and colours in the corresponding upper and middle panels. Right: (b1) Evolutionary tracks of four different scenarios of low-mass stars on the HR diagram. (b2) Asymptotic period spacing, ΔP, as a function of X_c. (b3) Profiles of hydrogen mass fraction X in the stellar interior for selected models, labelled by different symbols and colours in b1 and b2.