Mass constraints from the rms method for a potential additional planet in the HAT-P-18 b system. The yellow line traces the rms upper mass limit. Numbered points mark local minima (valleys) in this constraint where the period ratio P/P₀ of the perturber to the known transiting planet lies near a low-order (≤5) MMR; corresponding mass values are listed in the legend. The background color shows a dynamical stability map based on the MEGNO chaos indicator, with the scale provided by the color bar on the right.