Minimum initial inner binary separation min(a_in) to prevent RLOF at periastron at ZAMS (i.e., R₁(m₁)≤R_RL, 1 and R₂(m₂)≤R_RL, 2) as a function of the inner binary mass ratio q_in = m₂/m₁ and eccentricity e_in for HD 45485 (top), HD 191495 (middle), and HD 25631 (bottom). The red solid lines mark the present-day observed separation, corresponding to the evolved post-merger a_out in the merger-in-a-triple scenario, while the black area in the bottom left corners corresponds the unphysical region with initial inner binary separation with R_i(m_i)≤R_RL, i. In the bottom row, the boundary of this unphysical region has a kink caused by the change of slope in the assumed mass–radius relation: HD 25631 primary mass is M₁ ≃ 7 M_⊙, so for q_in ≲ 0.2 (the exact threshold depends on f_Δ), the expected values of m₂ are sufficiently low that the exponent ξ in the R_ZAMS ≡ R_ZAMS(m) changes. For the right column with f_Δ ≠ 0, our assumptions based on the Blaauw (1961) approximation may be inconsistent (see Sect. 5).