i′ − z′ vs. z′ colour − magnitude diagram for all GROND sources in the XMMU J0338.7 + 0030 region with flux detections in the i′,z′ bands at a significance level  ≥ 1σ, whatever the classification of the source by Le Phare (i.e., stars are included). Photometric errors (1σ) are also shown. Candidate members of XMMU J0338.7 + 0030 are reproduced with red (green) filled circles (squares) if they are within (outside) the bona fide cluster region (panel a)). The three spectroscopic cluster members in the GROND photometric catalogue are marked with blue filled circles there. Green asterisks represent the two QSO at a cluster-centric distance (CCD) greater than 1′. Alternatively (panel b)), photometric cluster members classified as galaxies are colour-coded according to their spectro-photometric type, which is E (red), Sbc(yellow), Scd (green), Im (cyan) or SB (blue). In each panel, the red short-dashed, solid and long-dashed lines represent the 1σ, 3σ, and 5σ flux thresholds of our photometry (cf. Sect. 2.3.2). The red dotted-short-dashed line represents the completeness limits in the i′,z′ bands, as obtained from Fig. 12. As a reference, the blue solid line reproduces the red-sequence defined by the inner regions of the 31 E + S0 + S0/a galaxies that are located within 1′ from the X-ray centroid of the cluster RDCS J0910 + 5422 at z = 1.106 (Mei et al. 2006a).