YHK colour − colour diagrams of sources with spectroscopic redshifts from the literature (GOODS-S field). The selection criteria (1) and (2) are shown by the dashed and solid horizontal lines respectively. Top: sources with spectroscopic redshifts from the GOODS-MUSIC catalogue (see legend for symbols) and stars from the Pickles 1998 stellar spectra flux library. We overplot early-type galaxies from Rettura et al. (2010) at z < 1.5 as well as the three passive galaxies at z > 1.8 from Cimatti et al. (2008) (green and orange triangles respectively). Bottom: Members of the overdensity at z ~ 1.6 found in GOODS-S (Kurk et al. 2009). Colours of the symbols account for the spectral class of the group members: red, green and blue for early-type, early-type with signs of star formation and late type galaxies respectively.