H-R diagram of the clusters with NGC 6791 in black, NGC 6819 in red and NGC 6811 in blue. The gray symbols indicate NGC 6819 shifted to the positions of NGC 6791 and NGC 6811, respectively. The dashed line roughly indicates the red-giant branch of NGC 6819. The dashed-dotted line indicates the position of the red-giant branch of NGC 6819 when only the mass is increased to 2.6 M_⊙ (NGC 6811). The dashed-dotted-dotted-dotted line illustrates the position of the red-giant branch of NGC 6819 when a mass of 2.6 M_⊙ and a metallicity of  − 0.35 dex are assumed (see text for more details).