(a) Adopted region of stable mass transfer in the primary mass-mass ratio plane. Regions coloured red experience unstable mass transfer and merge. For green regions, mass transfer is stable, and a Be star is formed. The black and orange lines show systems with secondary masses of 9 and 6 M_⊙, respectively. (b) Results of a Monte Carlo simulation showing the Be fraction, Φ_Be, when the stable mass-transfer region in (a) is applied. Binary systems have a flat mass-ratio distribution (κ = 0), a primary mass distribution ξ(M₁)  ∝ M^−1.9, and we have assumed inefficient accretion (ΔM/M_2, i = 0). The black line shows a simulation with a turn-off mass of 9 M_⊙, and the orange line a turn-off mass of 6 M_⊙. The dashed grey line shows the theoretical upper limit, as given by Eq. (30). Measured Be fractions of NGC 330 and NGC 2164 according to Fig. 3 are plotted as black and orange crosses, respectively.