New near-infrared observations of NGC 6528 are presented .
The JH K _ { s } observations complement a previous HST/NICMOS data set by Ortolani et al .
( [ ] ) , 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 _ { \it B - V } = 0.55 and ( m - M ) _ { \circ } = 14.44 ( 7.7 kpc ) .
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 ] \approx 0.0 , and [ Fe/H ] \simeq - 0.20 and +0.08 on the Zinn & West ( [ ] ) and Carretta & Gratton ( [ ] ) revised metallicity scale , respectively .
The best isochrone fit to the cleaned K,V - K diagram is obtained for a 12.6 Gyr 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 .
( [ ] ) 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 M 32 .