Central abundance of ¹H and ⁴He for V4142 Sgr. The mass-fraction inversion (X₁ stage) of the donor star (black line) is shown before the mass transfer (E stage) and mass inversion (U₁ stage). The gainer star evolves over a more extended timescale, during which it fails to achieve the mass-fraction inversion, and its ¹H increases slightly, by 0.04 M_⊙, during the accretion phase (E stage), allowing it to rejuvenate before the mass transfer concludes (H stage), as indicated by the green arrow.