Period of small oscillations about the resonance centre for a resonance with ϕ₄ or ϕ₁₉. Even though complete closed-form solutions exist (see Haponiak et al. 2020), the small-oscillation limit leads to handier formulas, suitable for order of magnitude estimates. The resonant angles are σ₄ = ψ + ϕ₄ and σ₁₉ = ψ + ϕ₁₉, respectively.Dashed curves are used for oscillations about Cassini state 2 before the separatrix appears. The appearance of the separatrix is marked by a blue dot.