J, J − K_s colour–magnitude diagram of the central field of the Arches cluster. L′-excess sources derived from Fig. 18 are marked as red diamonds. Proper motion members are shown in green, non-members are shown in red, and sources with unknown membership in black. The imposed magnitude limit to allow for proper-motion member selection, K_s< 17.5 mag, is shown as the dotted line. The horizontal dashed line marks the J-band completeness limit at J = 22 mag above which the combined cluster excess fraction was derived. The dashed vertical lines indicate the main-sequence colour selection applied in addition to the proper motion membership criterion to select the main-sequence reference sample to calculate the excess fraction. The location of the red diamonds near the main sequence shows most prominently that several L′-excess sources do not show excess emission at K_s. The J, J − K_s colour–magnitude diagrams of the outer fields can be found in Appendix (B.4).