Revisited parameters for the twin bulge globular clusters NGC 6528 and NGC 6553

Context. NGC 6528 and NGC 6553 are among the most metal-rich globular clusters in the Galactic bulge. They represent the upper end of the chemical enrichment in the Galaxy, and can increase our understanding of the processes of cluster formation and enrichment. Aims. Our aim was to refine the fundamental parameters of NGC 6528 and NGC 6553, based on proper motion-corrected Hubble Space Telescope WFC3 and ACS photometries. Methods. In order to derive the fundamental parameters of age, distance, reddening, and the total-To-selective absorption coefficient, we employed a Bayesian isochrone fitting. Age and metallicity are mainly constrained by the turn-off morphology, thanks to the unprecedented quality of the proper motion-cleaned photometry. Results. The two clusters show remarkably similar colour-magnitude diagrams. We derived an age of 11±0.5 Gyr with a solar metallicity for both clusters. The reddening for NGC 6528 and NGC 6553 is E(Ba V) = 0.63 and 0.76 and the distances from the Sun are d- = 7.85 and 5.1 kpc, respectively; we recall that the distances strictly depend on the adopted total-To-selective absorption parameter. Conclusions. The age of these metal-rich clusters is about 2 Gyr younger than the moderately metal-poor bulge clusters. The ages and metallicities are remarkably identical to the values of the bulk of bulge field stars.