Mg I EW versus ΣW_CaT for the Sextans, Sculptor and Fornax dSphs. The filled orange squares with error bars show the stars that are highly probable members (i.e. with velocities within  ± 2σ from the systemic velocity of the dSphs) and the black crosses the stars that are highly likely to be MW contaminants (i.e. with velocities at least 4σ away from the systemic velocity of the dSphs). The solid line indicates the theoretical relation derived in Sect. 2.