Upper panel: contours of the extended X-ray emission overlaid on the CFHTLS RGB image of the fossil group 101540 at z = 0.07. Middle panel: g′ − r′ versus z′. Lower panel: r′ − i′ versus z′. Filled black circles illustrate group members selected by the method described in Sect. 4.1. The BGG and second brightest satellite galaxy within 0.5R₂₀₀ have been marked with red asterisks within each color magnitude diagram. The upper and lower limits of colors are shown by horizontal dashed lines.