y₀ vs. (b − y)₀ colour–magnitude diagram with symbols as in Fig. 2. It is apparent that the selection of candidate cluster members in the c₁ and m₁ vs. (b − y) colour–colour diagrams (Figs. 2 and 3) is very efficient. To obtain a clean sample of RGB stars we use an additional CMD-based selection indicated by the red dashed lines. These were obtained by shifting a 13 Gyr, [Fe/H] = −2 dex, [α/Fe] = 0.4 dex Dartmouth isochrone from its best-fitting location by –0.04 and 0.13 mag along the colour axis in order to eliminate several AGB star candidates, as well as a handful of foreground and background contaminants.