1 Introduction

The Galactic bulge is a key component of our Galaxy. The study of the colour-magnitude diagram (CMD) of a Galactic bulge globular cluster is, therefore, valuable as it provides fundamental information about the cluster, such as its reddening, distance and chemical composition. These parameters play an important role in the understanding of formation and evolution of our Galaxy.

The Galactic bulge globular cluster NGC 6553 $\equiv$ GCL 88 ($\alpha_{1950} = 18^{\rm h} 06^{\rm m} 11^{\rm s}$, $\delta_{1950} = - 25\hbox{$^\circ$} 55\hbox{$.\mkern-4mu^\prime$}1$, $l= 5\hbox{$.\!\!^\circ$}253$, $b=-3\hbox{$.\!\!^\circ$}029$), 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$\hbox{$^\prime$}$ 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] = $-0.2\,\pm\,0.4$; 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 $\sim$ 0.1 to 0.2 mag in V up to $V\sim$ 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

(a)

presence of some peculiar features in CMDs of the cluster e.g. curved giant branch (GB), which Ortolani et al. (1990) attribute to strong blanketing in the atmosphere of the metal-rich stars of the cluster. They even locate the red giant branch (RGB) tip at a V-band brightness comparable to the horizontal branch (HB);

(b)

an anomalously tilted and very red HB; and

(c)

a nearly solar abundance for NGC 6553.

The cluster is projected against a rich stellar background affected by differential extinction but in the photometric studies carried out so far the RGB and the HB of the cluster were not corrected for field star contamination. To understand the above mentioned peculiar features in the CMD of NGC 6553 it is important to consider that the cluster may be at the upper end of the metallicity scale of Galactic globular clusters. In 1993 we obtained a new set of better quality VI CCD images for the cluster region as well as for an adjacent field region. While we were still analysing the observations, Ortolani et al. (1995) published HST observations of NGC 6533, showing unambiguously the main-sequence turn-off (MSTO) location. They estimated an age of 12 $\pm$ 2 Gyr, which places NGC 6553 among the old Galactic globular clusters. Although the present data are the deepest obtained so far from the ground for NGC 6553, they are definitely not reliable for determining the location of the MSTO, especially in the presence of the superior HST data. We have therefore not made any attempt either to derive the cluster age or to discuss about the location of MSTO from the present data.

In this work, VI CCD photometry for more than 40000 stars down to V = 23 mag in a field of about $6 \times\linebreak 10$ 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.