Colour–magnitude diagrams of the clusters studied (small grey dots). The literature sources for the photometric data are given in the text. Filled circles are giant stars in the RGB or RC belonging to the cluster based on their radial velocities and thus used in the metallicity determination. The open star is a main sequence star in Berkeley 23 based on its radial velocity, but it has not been used in our metallicity analysis. Open circles are stars observed but classified as non-members based on their radial velocities (see Sect. 3). Isochrones derived by Pietrinferni et al. (2004) have been plotted as reference using the values listed in Table 1 and the metallicities derived in this paper. The RC position is covered by the observed stars.