Spectroscopic Hertzsprung-Russell diagram of a possible LB-1 progenitor. The helium abundance of the donor is indicated by colour. The mass gainer, which does not leave the main sequence as the binary model ends when the primary becomes a WD, is shown in grey. The ZAMS parameters are M₁ = 4.0 M_⊙, M₂ = 3.5 M_⊙, and P_orb = 16 d. After the RLO the parameters were M₁ = 0.7 M_⊙, M₂ = 3.5 M_⊙, and P_orb = 223 d. The surface abundances of the donor after RLO are shown in the lower left corner. The star symbols indicate Roche-lobe decoupling, helium ignition, middle of helium burning phase (sdOB observationally), and central helium depletion. The observations of Shenar et al. (2020) and Lennon et al. (2021) are shown in black and grey.