Mass and semi-major axis of a possible undetected planet between TOI-411 b/c (upper panel) and TOI-411 c/d (lower panel). The minimum semi-major axis (a_xmin) is shown as a dashed line and the maximum one (a_xmax) is shown as a continuous line for Δ_crit = 10 and 12.3 in red and green, respectively. Given Δ_crit, the shaded region contains the possible combinations (M_x, a_x) for a stable system configuration hosting one more planet.