Comparison of the CO core masses at the end of core He burning between primaries in binary models (M_CO) and single stellar models (M_CO; single). Each binary evolution model, defined by the initial primary mass (M₁) and the initial orbital period (P_ini), is represented by a rectangle colored according to the ratio M_CO/M_CO; single. The initial mass ratio is q = 0.6. Single stellar models correspond to P_ini > 10⁴ days. Similarly to Fig. 6, areas in the background indicate the evolutionary state of the donor stare at the onset of RLOF. We find that the CO core masses in low-Z binary models can be very close to those formed in single stars, despite the mass-transfer interaction. This is because only partial envelope stripping occurs in those models.