Period, surface helium abundance, and luminosity-to-mass ratio during the contraction phase after RLO, when the stripped star has obtained an effective temperature of 12 500 K (close to that derived for LB-1). The black and grey symbols indicate the values for LB-1, as derived by Shenar et al. (2020) and Lennon et al. (2021, almost superimposed in the colour bar). The fiducial model is shown in red. The orange curve (initially 4.27 M_⊙ + 4.07 M_⊙ at P_orb = 24.2 d) shows the surface helium evolution of a mass donor, which does not develop convection in its outer layers during RLO. In the magenta curve (initially 5.86 M_⊙ + 4.20 M_⊙ at P_orb = 25.4 d) convection sets in.