Internal structure of a 1.5 M_⊙ pre-CE RGB star at the epoch when the H-free core is 0.3202 M_⊙ (plotted versus the Lagrangian mass coordinate m_r). Shown are the H abundance by mass, X_H(m_r), and the cumulative H mass interior to m_r, M_H(< m_r), which increases outwards. Vertical dashed lines mark the BP criteria (X_H = 0.1, peak nuclear energy generation, and maximum compression). The dashed green line marks M_H = 1.0 × 10⁻⁷ M_⊙, the H content inferred for the secondary of J2102–4145 from the observed (R, T_eff). The plot shows only the innermost envelope layers of the RGB progenitor, which set the residual H content.