Schematic comparison of orbital shrinkage via common envelope and stable mass transfer in a BBH merger progenitor (here a binary with a 35 M_⊙ stellar donor and 10 M_⊙ BH accretor), based on a simple analytical model (Sect. 2.3) rather than a detailed evolutionary calculation. The SMT lines follow assumptions typically used in population synthesis of GW sources, naively suggesting that mass transfer with enhanced AM loss (blue) can produce separation as small as CE evolution. In this paper, we use detailed binary evolution calculations to show why this is not the case (Fig. 3).