CMFGEN model spectrum computed at the beginning of the BSG stage using outputs from our MESA binary models for a representative system of a 20 M_⊙ BSG primary and an 18 M_⊙ (MS) secondary. The red line profiles indicate the CMFGEN model spectrum of a primary star with T = 23 000 K and log(g) = 3.0 dex. The green line indicates a combination of the model spectrum of the primary star with T = 23 000 K and log(g) = 3.00 dex and the secondary star with T = 26 000 K and log(g) = 3.50 dex. The flux ratio of the primary to the secondary in the B band at the stage when the spectra are computed is F_B,  pri/F_B,  sec = 2.3. Top panel: spectral region around the Hδ line which is one of the diagnostics for log g. Bottom panel: spectral region around SiII lines at 4553, 4568 and 4575 Å, which are diagnostics along with SiII and SiIV lines.