Distribution of all sources with fluxes in the GROND g′,z′,Ks channels above the respective 1σ thresholds in the z′ − Ks vs. g′ − z′ colour − colour diagram. Magnitudes within a 3′′-wide circular aperture are used. Sources identified as point-like on the basis of a SExtractor stellarity index larger than 0.98 are marked with red asterisks. Conversely, extended sources within (beyond) 1′ from the position of XMMU J0338.7 + 0030 are marked with filled (empty) circles. The short-dashed and solid lines represent, respectively, the computed and adopted thresholds to select bona fide stars (see text).