Resulting BH masses and spins of CHE stars using a semi-analytical model (see Sect. 3). The model assumes a mass ratio of unity, so two BHs with identical properties are produced. We consider the outcome for three masses at TAMS (25 M_⊙, 40 M_⊙, and 60 M_⊙) and three orbital periods at TAMS scaled from the period at which the binary would be in contact. Each line represents the results for a fixed M_TAMS and P_TAMS with a variable f_WR (where M_BH = f_WRM_TAMS). Cases where the resulting BBH would be too wide to merge within the age of the Universe are marked with a dotted line. Note that for f_WR close to unity, lines with different values of P_TAMS converge to the same spins at any given mass. The effect of (P)PISNe is ignored in this figure. Precise boundaries depend on the metallicity used to determine stellar properties in the semi-analytical model used, which in this case were taken from calculations at Z_⊙/50.