MEGNO (⟨Y ⟩) stability map for the WASP-18 system. We over-plot the map with an upper mass of a hypothetical perturbing planet introducing a mid-transit time TTV_RMS scatter of 47 s (solid line) as obtained in this study. The stipulated line is the upper mass limit as obtained from the RMS scatter (30 m s ⁻¹) of the radial-velocity curve. For initial conditions resulting in aquasi-periodic (i.e., bounded) motion of the system, the ⟨Y ⟩ value is close to 2.0 (color coded blue). For chaotic (i.e., unstable) motion, the ⟨Y ⟩ is diverging away from 2.0 (color coded red to yellow). Vertical arrows indicate (P₂ ∕P₁) orbital resonances between the perturbing body and the transiting planet. The two planets were assumed to be co-planar, and the eccentricity of the perturbing planet was initially set to zero.