Position of the whole studied sample of BSGs (left panel) on the Hertzsprung–Russell diagram together with non-rotating single star evolutionary tracks for the SMC metallicity (Schootemeijer et al. 2019). The given tracks were calculated by adopting the efficiency of semiconvection (α_sc) equal to 1 and the convective core step overshooting efficiency (α_ov) equal to 0.33. The right panel represents the position of only “bona fide” spectroscopic binary BSGs for which prominent periods of RV variations were constrained. In addition, four evolutionary tracks with adopted α_ov = 0.55 were added for reference (black solid lines).