Abstract details

The Galactic globular cluster (GGC) NGC 6441 presents several peculiar features: a high metal abundance and an extended Blue Horizontal Branch together with a sizable sample of RR Lyrae stars. Moreover, the mean period of fundamental RR Lyrae is $\approx$ 0.759 days, similar to those found in metal-poor GGCs. This empirical evidence casts some doubts on the universality of the V-band metallicity relation of RR Lyrae stars. Accurate estimates of mean V-band magnitudes of RR Lyrae in NGC 6441 are hampered by differential reddening, since this system is located at low Galactic latitudes. To constrain the volutionary status of these objects we plan to use the K-band Period-Luminosity-Metallicity (PLZK). Theoretical and empirical circumstantial evidence indicate that this approach is marginally affected by evolutionary effects, reddening and presents a mild dependence on the stellar mass. We have already colleted a good sample of NIR (J,K) images of NGC~6441. With these data we plan to supply a robust estimate of its distance. Moreover, we plan to discuss evolutionary properties from the tip of the RGB down to the TO region. Note that the use of optical and NIR data will allow us to split cluster and field stars.