a) DSS–R image of the ~19′× 19′ area around M0520 (same as in Fig. 1, Right panel); dashed circles have radii r₅₀₀ and are centred on the X-ray centre of each cluster. The red small circles mark the galaxies selected in the range R = 17.8–21.0; the large black circles show only those galaxies within the range of colours R − 3.4 μm = 3.5–4.5. b) R − 3.4 μm CMD of the galaxies within the ~19′× 19′ area as in panel a) red and black circles as in panel a). c) R − 3.4 μm CMD of the galaxies within the dashed circles in panel a) the crosses are all the galaxies in the range R = 17.8 − 21.0, the filled dots are the colour-selected once (R − 3.4 μm = 3.5–4.5) .The BCG of M 0520 and the optical counterparts of radio sources S1 and S2 are marked by magenta circles (they are also highlighted by magenta arrows in panel a), for clarity).