Gas mass fraction as a function of total mass computed within r₂₅₀₀ (upper panel), and within r₅₀₀ (middle panel); the differential gas mass fraction, [M_gas,500 − M_gas,2500]/[M_tot,500 − M_tot,2500], as a function of M_tot,500 (lower panel). The triangles represent the nine groups, while the circles represent the 114 clusters from M12 (we show in blue the strong cool-core clusters). The red lines show the best fits for the entire sample. The dashed line in the middle panel corresponds to the best fit found by Laganá et al. (2011).