The Galactic bulge globular cluster NGC 6553 GCL 88 (, , , ), located in the direction of the Galactic centre belongs to metal-rich globular clusters (Zinn 1985). Structurally, it is a compact cluster with compactness parameter, c=1.17 but it does not appear to be post-core-collapsed (Trager et al. 1995). It has a core radius of about 0.6 and radial velocity of -27 km s-1. An inspection of sky survey plates indicates that the cluster is in a patchy and heavily obscured zone. Reddening determinations for the cluster vary from E(B-V)=0.7 to 1.0 mag, while its estimated distance ranges from 3 to 6 kpc (Hartwick 1975; Ortolani et al. 1990, 1995; Guarnieri et al. 1998). The metallicity of NGC 6553 is close to solar ([Fe/H] = ; Barbuy et al. 1992; Bruzual et al. 1997; Guarnieri et al. 1998 and references therein). Radial velocity and distance parameters of the cluster indicate that it is indeed a member of the Galactic bulge.
Hartwick (1975) presented a photographic BV diagram with an accuracy of 0.1 to 0.2 mag in V up to 18 mag. Lloyd Evans & Menzies (1973) obtained instrumental VI photographic diagram for the cluster. But the usefulness of photographic data in such a crowded stellar field is very limited. Ortolani et al. (1990) have done a detailed BVRI CCD photometric study of the cluster by observing about 15000 stars down to V = 21.5 mag. They derived the cluster parameters and also indicated the presence of differential extinction across the cluster face. Recently, Guarnieri et al. (1995, 1998) have studied the cluster by combining the optical data with near-IR photometry. These studies indicate
In this work, VI CCD photometry for more than 40000 stars down to V = 23 mag in a field of about arcmin2 is presented. The data have been used to derive photometric parameters of the cluster and to present, for the first time, a cluster sequence corrected for the field star contamination. Reddening and distances to the field populations present in the direction of NGC 6553 are also estimated for the first time. The observations, data reduction and determination of metallicity, reddening and distance are described in the sections to follow.
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