Position of branch 1 of elliptic libration centres of the complete Hamiltonian (3) in the resonance 1:1:2 for −7 ≤ δ ≤ 7. The planetary masses are the same as in Table 1. As a comparison, branch 1 of equilibria of the simplified Hamiltonian (25) is plotted in grey for the same range in δ. For small enough values of δ the eccentricity is not too high and the agreement is good. The simplified and the complete Hamiltonian diverge when δ → +∞.