Locus for the formation of a GW source through stable MT with 40 M_⊙, 32 M_⊙, 25 M_⊙ donors, i.e. BH progenitors, as a function of the CO accretor mass., m_a, i, and the orbital period, P_i. Different colors mark the favorable strips for merging double COs via purely isotropic re-emission (β = 1) and mass outflow from L₂ with variable efficiencies υ >  0, keeping β + υ = 1. Solid black lines mark the range of known NS masses from binary systems (Tauris et al. 2017).