CMFGEN model spectra computed at the end of the BSG stage using outputs from our MESA binary models, taken from the same binary models as in Fig. 7, but at a later evolutionary stage. The red line profiles indicate the CMFGEN model spectrum of a primary star with T = 9650 K and log(g) = 1.50 dex. The green line indicates a combination of the model spectrum of the primary star with T = 9650 K and log(g) = 1.50 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 below the Balmer jump is 3.0. Top panel: Hδ line for the primary (red) and combined spectra (green). Middle panel: Balmer jump for the primary (A-type supergiant; red) and combined spectrum (green). Fluxes are normalised at 3790 Å for clarity. Bottom panel: change in Balmer jump in the combined spectrum relative to the primary spectrum, as a function of initial primary mass. The combined spectrum has the lower Balmer jump.