Observed colour–colour diagram, g − i as a function of u − g for the galaxies in the CL1301.7−1139 region. Our ALMA sample is shown in green, with squares for CL1301.7−1139 and with circles for CL1411.1−1148. In both cases, pink borders indicate star-forming galaxies that have low gas fractions for their stellar masses (see Sect. 4.1). The small black dots show galaxies with spectroscopic redshifts within 3 × σ_cl of the CL1301.7−1139 redshift. Galaxies that were not detected in CO are indicated with a cross.