Equivalent width of the lithium line as a function of T_eff. The purple stars are the known members of the four clusters used to trace the lower envelope of the bulk of cluster spectroscopic candidates (solid line), as given in the literature. The open red circles show the members of each cluster that were reported in the literature and observed again by GES. The green symbols mark the stars selected as spectroscopic candidates. The stars excluded by the lithium criterion are shown in black. We also highlight the sources with an estimate of EW(Li) (dots and squares) or only an upper limit (arrows and crosses).