Quadruple-star evolution. In each panel the upper two rows show the eccentricity and minimum periastron separation of the Be star progenitor binary, and the lower two rows show the eccentricity and minimum periastron separation of the observed B+B binary, all as a function of the initial inclination of the Be star progenitor binary. The initial inclination of the B+B binary is i_A = 20^∘ in the left and middle panels and i_A = 50^∘ in the right panel. The initial eccentricity of the B+B binary is e_A  =  0.02 (left and right panels) and e_A  =  0.27 (middle panel). The green shaded region denotes that the Be star progenitor binary merges or is a merger candidate, while the hatched region denotes that the B+B binary merges or is a merger candidate.