1 Introduction

Color-magnitude diagrams (CMDs) of globular clusters (GCs) can be subdivided into three main groups according to the appearance of their horizontal branches (HBs). The morphology of a HB can be numerically expressed using either the parameter B/(B+R) (Rosino 1965; Mironov 1972) or the parameter $R_{\rm HB}=(B-R)/(B+RR+R)$ (Lee et al. 1988), where B and R are respectively numbers of HB stars to the blue and to the red of the instability strip and RR is the number of RR Lyrae variables. Figure 1  shows the distribution of the GCs of our Galaxy over $R_{\rm HB}$; the parameter values are from the catalog by Harris (1996).

  

Figure 1: The distribution of the globular clusters of our Galaxy over the morphological parameter of the horizontal branch, $R_{\rm HB}$

The first group consists of clusters with "blue'' HBs, with all HB stars to the blue of the RR Lyrae instability strip. The values of B/(B+R) and $R_{\rm HB}$ for these clusters are close to 1. These are, as a rule, very metal-poor GCs of the galactic halo, with typically $[{\rm Fe/H}]=-1.5$ to -2.1.

The second group consists of clusters with "red'' HBs, with all HB stars to the red of the RR Lyrae instability strip. For such clusters, B/(B+R)=0, and $R_{\rm HB}=-1$. Usually these are clusters of the Galaxy's disk or central region, with typically $[{\rm Fe/H}]=-0.5$ to -1.0.

Finally, the third group contains GCs with HBs populated on both sides of the instability strip. This group is very heterogeneous both by HB morphology as well as by metallicity. Studying these clusters may give us a key to the "second parameter'' problem.

The cluster studied in this paper, NGC 6723 ($18^{\rm h}59\hbox{$.\!\!^{\rm m}$}6$, $-36^\circ38'$, 2000.0), is one of the clusters of the third group. It is a southern globular cluster located towards the galactic center, at about 2.4 kpc distance from the galactic plane, in a field with low interstellar reddening: the catalog of Harris (1996) gives E_B-V= 0.05.

The first photographic photometry of stars in NGC 6723 was published by Gascoine & Ogston (1963). More thorough studies were later undertaken by Menzies (1974) and Martins & Fraquelli (1987). The first estimates of the cluster's integrated spectral type (Kinman 1959; Kron & Mayall 1960) were G2-G4. Smith (1981), from the spectroscopic metal-abundance parameter $\Delta S$ derived for five RR Lyrae stars, found $\Delta S=2.8$ for NGC 6723, corresponding to $[{\rm Fe/H}]=-0.68$ in the metallicity scale of Butler (1975). However, later studies showed a more significant metal deficiency. Smith & Hesser (1986), from DDO photometry, determined $\rm [Fe/H]=-1.1$. A similar value, $\rm [Fe/H]=-1.1$,was found by Zinn & West (1984) and Lee (1992). Recently Fullton and Carney (Fullton & Carney 1993; Fullton & Carney 1996; Fullton 1996) obtained 2-color $(B,\,V)$ CCD photometry of almost 18000 NGC 6723 stars, down to the 24th magnitude; the results of this study have not yet been published in the complete form. Fullton and Carney also obtained spectra ($\lambda \lambda 5000-7800$ Å) of three red giants in NGC 6723, calibrated them with model atmospheres of Kurucz (1993), CD-ROM and, as a result, obtained still lower metal abundance, $[{\rm Fe/H}]=-1.26$. Harris (1996) catalogs $[{\rm Fe/H}]=-1.12$ (in the Zinn & West 1984, scale), along with F9 for the cluster's integrated spectral type.

The present study is the first deep five-color study of NGC 6723. It enabled us to determine a number of important parameters for this cluster.