Initial convective core mass of mass donor (M_ccd, i thick black line) and initial accretor mass M_a, i (thin coloured lines) for five initial mass ratios q_i = M_a, i/M_d, i (0.2, 0.4, 0.6, 0.8, 1.0) as a function of the initial donor mass M_d, i in massive binary systems. For highly non-conservative mass transfer, reverse Algol evolution cannot occur above the thick black line (yellow region), only below it (blue region), whereas for conservative evolution it cannot occur above the yellow line. The symbols mark the positions of VFTS 094 and VFTS 176 in this diagram, assuming inefficient (blue) or conservative (yellow) mass transfer, with the ordinate value giving their initial accretor masses. The black dashed line shows the limiting initial companion mass above which the models of Pauli et al. (2022) that avoid a contact phase, also avoid merging during Case A mass transfer. The dash-dotted line shows the limiting initial companion mass below which all models merge. Between these two lines, whether the models merge or not is a function of their initial orbital period, and the fraction of models that merge increases for lower initial accretor masses. Lighter shading is used to indicate that not all models avoid merging. It is also assumed that all binaries with q_i < 0.2 (grey line) merge before entering the Algol stage.