Evolution of the donor star size and effective temperature in V4142 Sgr. Left: evolution of the donor star size with an initial mass of 3.5 M_⊙ in the binary system DPV V4142 Sgr, with a companion that has an initial mass of 1.5 M_⊙. We show the phases in which cases A, B, and C of RLOF can occur, depending on the age of the system. The inversion of the ¹H/⁴He ratio (X₁ stage) is represented by a brown dot, the mass transfer (E stage) by a blue dot, and the current stage of the system (X₂ stage) by a red dot. Right: effective temperature, T_eff, as a function of age (in years) for V4142 Sgr. We shown that the donor star, after crossing the Hertzsprung gap during the onset of the blue loop (stages C and D), follows an evolutionary track until reaching its current stage (X₂ stage) on a thermal timescale. This continues until it reaches the final stage of non-optically thick mass transfer (H stage), stopping shortly before helium depletion.