ABSTRACT. We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6441 using time-series BV photometry. The total number of variables found near NGC 6441 has been increased to ~104, with 48 new variables being found in this survey. A significant number of the variables are RR Lyrae stars (~46), most of which are probable cluster members. As was noted by Layden et al. in 1999, the periods of the fundamental-mode RR Lyrae stars are unusually long compared with field stars of similar metallicity. The existence of these long-period RRab stars is consistent with Sweigart & Catelan's prediction that the horizontal branch of NGC 6441 is unusually bright. This result implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal. We discuss the difficulty in determining the Oosterhoff classification of NGC 6441 because of the unusual nature of its RR Lyrae stars. A number of ab-type RR Lyrae stars are found to be both brighter and redder than the other probable RRab found along the horizontal branch, which may be a result of blending with stars of redder color. A smaller than usual gap is found between the shortest period fundamental-mode and the longest period first-overtone–mode RR Lyrae stars. We determine the reddening of the cluster to be E(B-V) = 0.51 ± 0.02 mag, with substantial differential reddening across the face of the cluster. The mean V magnitude of the RR Lyrae is found to be 17.51 ± 0.02 mag, resulting in a distance of 10.4–11.9 kpc, for a range of assumed values of M_V for RR Lyrae stars. The possibility that stars in NGC 6441 may span a range in [Fe/H] is also discussed.

Key words: globular clusters: individual (NGC 6441); RR Lyrae variable

An erratum for this article has been published in 2003 The Astronomical Journal 125 2750

Print publication: Issue 5 (2001 November)
Received 2001 April 9, accepted for publication 2001 July 16