Stability of the HD 108874 system. The values of the semi-major axes of the two planets are sampled around the nominal system (black square) according to the dispersion of the main secular frequency. The black box marks the 1σ uncertainties given in Table 4. The three yellow strips indicate the instability zones related to the 5:1, 9:2 and 4:1 MMR, respectively. A logarithm scale is used for the colour coding, where small values of the dispersion mean stability, while large values imply fast changes of the secular frequency and then chaotic evolution.