One-degree of freedom model of the 2:1 MMR (Henrard & Lemaitre 1983; Deck et al. 2013). We note that X and Γ are functions of the orbital elements and the masses of the system. The term Γ represents how deep the system is in the resonance, while X parameterizes the position of the fix points and separatrices. The blue dots represent the intersection of 10 000 randomly selected samples of the posterior with the X-Γ plane.