HR diagram position of the donor stars at the onset of a CE event with a BH accretor at Z = 0.1 Z_⊙ (left) and Z_⊙ (right), with six evolutionary tracks for single stars at M_ZAMS = 20, 40, 60, 80, 100, and 150 M_⊙ for M0 (black dashed line) and M0_RMAX (black solid line). The gray dots represent the CE events with a BH companion that lead to a stellar merger and the potential formation of a QS, while the blue and green dots represent respectively HG and CHeB stars (i.e., evolutionary types in StarTrack, following Hurley et al. 2000) in binaries that successfully expel the CE and become BH-BH. The red and blue lines represent the threshold for respectively M_α_ML1.5 and M_α_ML1.82_MLTpp, where a star becomes cold enough to develop a convective envelope. The horizontal purple dotted line represents the luminosity threshold beyond which we extrapolate the convective boundary tracks. We plot the distributions of log T_eff and log L values of donors that have led to the CE mergers (in gray) or to CE ejections and the formation of BH-BH systems (in green or blue, depending on the donor evolutionary type in StarTrack). In the right panel there is a near-complete overlap between the BH-BH progenitor binaries and the ones that become GW sources. The dashed-line histograms correspond only to BH-BH systems that merge within a Hubble time.