Mem. S.A.It. Vol. 79, 355 c  SAIt 2008 Memorie della M. Dall’Ora 1 , P. B. Stetson 2 , G. Bono 3,9 , J. Storm 4 , G. Andreuzzi 5,3 , R. Buonanno 6 , F. Caputo 3 , M. Marconi 1 , M. Monelli 8 , A. Piersimoni 7 , V. Ripepi 1 , and L. Vanzi 9 1 INAF - Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131, Napoli, Italy, e-mail: dallora@oacn.inaf.it 2 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, Victoria, Canada 3 INAF, Osservatorio Astronomico di Roma, Italy 4 Astrophysikalisches Institut Potsdam, Germany 5 INAF - Fundacion Galileo Galilei, La Palma, Spain 6 Universit` a di Roma Tor Vergata, Roma, Italy 7 INAF, Osservatorio Astronomico di Collurania, Teramo, Italy 8 Instituto de Astrofisica de Canarias, Tenerife, Spain 9 European Southern Observatory Abstract. We present optical- near infrared photometry of the peculiar Galactic globu- lar cluster NGC 6441. By using the K-band Period-Luminosity-Metallicity (PLZK) of its RR Lyrae stars, we supply a new estimate of its distance. With this distance, we discuss the comparison in the K-( J − K) plane between observations and ”canonical” evolutionary models. Key words. (Galaxy:) globular clusters: individual (NGC 6441) -- stars: horizontal-branch – stars: variables: other – techniques: photometric 1. Introduction The Galactic Globular Cluster (GGC) NGC 6441 presents several interesting features: a high metal abundance ([Fe/H] = −0.53) and an extended Blue Horizontal Branch (EHB) — at odds with the general trend observed in the Galaxy and with the expectations of the canon- ical evolutionary models — and a tilted Red HB. Observationally, accurate photometric studies on NGC 6441 are hampered by Send offprint requests to: M. Dall’Ora strong differential reddening, serious field contamination, and high central concentration. It is worth noting that the differen- tial reddening, alone, does not seem to explain the Horizontal Branch morphology (Raimondo et al. 2002), nor has any mean- ingful spread in the metallicity been found (Gratton et al. 2007). Therefore, to explain both the EHB and the tilted Red HB, two different populations have been proposed (Busso et al. 2007; Caloi & D’Antona 2007), one with a ”canonical” helium content of Y ∼ 0.26, and one with helium enrichment up to