1 Introduction

A major question in stellar population studies concerns the transition regions between bulge, thick disk and halo of the Galaxy. Globular clusters are tracers to investigate this question. In particular the formation scenario and extent of the bulge are not yet established (e.g. Armandroff 1989; Larson 1990; Minniti 1995; Spergel et al. 1996).

CCD data in the last decade are forming a more consistent picture of the spatial distribution and intrinsic properties of clusters in the central regions of the Galaxy.

The properties of 16 out of 17 known clusters within $5^\circ$ of the Galactic center were gathered and discussed in Barbuy et al. (1998a). In that study it became clear that the inner bulge includes a non-negligible fraction of blue horizontal branch (BHB) clusters. A study in a wider region as far as $r \approx 20^\circ$ is necessary in order to understand the distribution of these populations of globular clusters and their membership to the halo or bulge. Minniti (1995) concluded, from density distribution models, that a disk component is not consistent for the inner parts of the globular cluster system, and he predicts the ratio of bulge/halo clusters as a function of distance. At 0.7 kpc ($r \approx 5^\circ$ at the Galactic center) $68\%$ of the globular clusters should belong to the halo, while at 3 kpc ($r \approx 20^{\rm o}$) the expected fraction is 87%.

In the present paper we study Terzan 9 ($r=4.1^\circ$ from the Galactic center), which is the last cluster within the $5^\circ$ region, still missing a deep CMD, as well as NGC 6139 and NGC 6453, which are, in turn, part of the sample in the surrounding ring $5^\circ<r<20^\circ$, located at $r=18.9^\circ$ and $5.8^\circ$ respectively.

As usual in the central parts of the Galaxy, two of the sample clusters, Terzan 9 and NGC 6453 have post-core-collapse structure, which is not the case of NGC 6139, however still a concentrated cluster with c=1.80 (Trager et al. 1995).

In Sect. 2 the observations are described. In Sects. 3, 4 and 5 the clusters Terzan 9, NGC 6139 and NGC 6453 are analysed, respectively. The concluding remarks are provided in Sect. 6.