z − H vs. H colour − magnitude diagram for the region encompassing XMMU J0338.7 + 0030 imaged with OMEGA2000. Red filled circles represent galaxies within 45′′ from the X-ray position of the cluster. The blue dashed lines reproduce the 50% completeness of the photometric catalogue extracted from the OMEGA2000 data. The horizontal red short-dashed line represents the z − H colour expected for an SSP model corresponding to the evolutionary phase of an instantaneous burst of star formation observed at z = 5 with Salpeter (1955) stellar IMF and solar metallicity observed at z = 1.45. This SSP model describes the average colour of the seven galaxies associated with XMMU J0338.7 + 0030 that exhibit 2.8 ≤ z − H ≤ 3.8   Vega   mag. The vertical red dotted line represents the characteristic magnitude H^⋆ at z = 1.45 (from Fassbender et al. 2007). Empty black squares mark the four spectroscopic cluster members that were successively established to be at z = 1.1 through VLT-FORS2 observations (see Fig. 11 and Table 1 in Sect. 2.4.2); blue filled circles highlight those with ID  =  12 and ID  =  15, which are at a cluster-centric distance greater than 45′′. The two spectroscopic cluster members without convincing evidence of ongoing star-formation activity (with ID  =  2 and ID  =  16) exhibit about as red z − H colours as the previous SSP model (see Appendix B, however).