Colour−magnitude diagram in the field of MS 1621.5+2640. The black points are the galaxies located at r< 500′′ from the cluster centre. The red dots are galaxies with r< 200′′ and a z_phot compatible with the cluster redshift. The straight line is the best fit to the red sequence and the dashed lines are the 3σ limits of the red sequence. At the cluster redshift (z = 0.43), the expected colour, computed with the synthetic evolutionary code of Bruzual & Charlot (2003), is r′−i′ = 0.77.