2003A&A...402..607M

New near-infrared observations of NGC 6528 are presented. The JHK_s observations complement a previous HST/NICMOS data set by Ortolani et al. (2001A&A...376..878O), in that they sample a larger area, contain a more numerous sample of red giant stars, and include the K band. Also, archival HST data sets (separated by 6.093 years) were used to proper-motion decontaminate the near-infrared sample and extract a clean VJHK catalogue. Using the present wide colour baseline, we compared the cleaned colour-magnitude diagrams of NGC 6528 with those of NGC 6553 and NGC 104 and derived new estimates of reddening and distance, E_B–V=0.55 and (m-M)₀=14.44 (7.7kpc). Moreover, the morphology and location of the cleaned red giant branch were used to derive a photometric estimate of the cluster metallicity. The average of 10 metallicity indicators yields a mean value of [M/H]≃0.0, and [Fe/H]≃-0.20 and +0.08 on the Zinn & West (1984ApJS...55...45Z) and Carretta & Gratton (1997A&AS..121...95C) revised metallicity scale, respectively. The best isochrone fit to the cleaned K,V-K diagram is obtained for a 12.6Gyr and Z=0.02 isochrone, i.e. the derived metallicity of NGC 6528 turns out to be very close to the mean of stars in the Baade's Window. Five AGB variable star candidates, whose membership has to be confirmed spectroscopically, are bolometrically as bright as the known long period variable stars in NGC 6553. As discussed in Guarnieri et al. (1997ApJ...477L..21G]) for NGC 6553, this may indicate that an ``intermediate age'' population is not needed to account for the brightest stars in external galaxies such as M32.