Colour–magnitude diagram (r′−i′,r′) of galaxies in a region of 12.4′ × 9.6′. Red symbols correspond to galaxy members confirmed spectroscopically (red dot corresponds to the BCG of the cluster). The solid line represents the red sequence defined as the densest locus in this diagram, which follows the linear fit r′−i′ =  − 0.028 * r′+1.118. Dashed lines delimit the region that encloses the RS and the blue cloud in this diagram. Galaxies included in this region are considered likely members, which are used to obtain the isodensity galaxy distribution shown in Fig. 1.