Color–magnitude diagram of the three open clusters NGC 6791 (blue squares), NGC 6819 (red circles), and NGC 6811 (green triangles), with color and magnitudes of the 60 cluster RGs sourced from Stello et al. (2011), Corsaro et al. (2012). Hydrogen-shell-burning and core-He-burning RGs are shown with filled and open symbols, respectively, with an evolutionary stage identified according to Corsaro et al. (2012, 2017), and to Molenda-Żakowicz et al. (2014) for the two stars KIC 9776739 and KIC 9716090. Isochrones are shown for each cluster as solid lines (see Stello et al. 2011, for more details).