1 Introduction

It is important to establish the nature of faint star clusters in the Galaxy, in order to better understand the Galactic structure, and the number and properties of objects related to the different subsystems (disk substructures, bulge and halo). In the later years we have been able to determine the properties of many misclassified objects. Some open cluster candidates turned out to be globular clusters, such as Lyngå 7 (Ortolani et al. 1993), NGC 6540 (Bica et al. 1994), or vice-versa AM-2 (Ortolani et al. 1995) and ESO 93-SC08 (Bica et al. 1999), both among the oldest open clusters. An interesting case was that of NGC 6749, previously included in both categories, globular and open clusters, in the latter case referred to as Berkeley 42. As unveiled by means of deep Colour-Magnitude Diagrams (CMD), it is a metal-poor globular cluster (Rosino et al. 1997; Kaisler et al. 1997). Another case is IC 1257, previously classified as open cluster, and now shown to be a compact halo globular cluster (Harris et al. 1997). The faint star cluster UKS 2, also designated as UKS 0923-545, IAU 0923-545, BH 66 and ESO 166-SC11 is located in Vela at $l = 276.003^\circ$, $b = -3.008^\circ$ (J2000.0 $\alpha$ = 9$^{\rm h}$25$^{\rm m}$17${\rm s}$ and $\delta = -54^\circ$43'10''). The designation UKS 2 arises from its detection on infrared plates taken with the 1.2 m UK Schmidt telescope, likewise the extremely reddened bulge globular cluster UKS 1 (Malkan et al. 1980; Ortolani et al. 1997). The first photometric study of UKS 2 was that of Malkan (1982). The cluster is also present in the survey by van den Bergh & Hagen (1975) as BH 66, where they reported an angular diameter of 3.5' and described it as a moderately rich cluster by means of B and R Cerro Tololo Curtis-Schmidt telescope plates. In the fourth list of the ESO/Uppsala survey (ESO B Atlas) of the southern sky (Holmberg et al. 1977; Lauberts 1982) it was classified as an open cluster with an angular diameter of 2'.

UKS 2 has also been included in several globular cluster studies. Malkan (1982) derived a reddening E(B-V) = 0.7 from integrated infrared photometry. Zinn (1985) estimated a metallicity [Fe/H] = -0.29 using Malkan's infrared photometry. Webbink's (1985) compilation lists E(B-V) = 0.74 and [M/H] = -0.37, based on the same data sources. Using magnitude estimates of bright giants, Webbink obtained a horizontal branch level $V_{\rm HB}$ = 17.75, implying a distance from the Sun $d_{\rm\odot}$ = 9.0 kpc. Armandroff (1988) presented a V CCD image of the cluster and pointed out that its appearance resembled that of an open cluster rather than that of a globular cluster. The cluster is poorly populated and rather loose to be considered a globular cluster, with a concentration parameter c = 0.66, as can be estimated from the core and limiting radii listed in Webbink (1985). No CMD is available for the cluster.

Recently Bica et al. (1998) studied an integrated near-infrared spectrum of UKS 2, deriving E(B-V) = 0.20, a metallicity of [ $Z/Z_{\rm\odot}$] = -0.15 and an age of 1 Gyr.

The aim of the present study is to define the nature of this cluster and derive its parameters based on CMDs. In Sect. 2 the observations are described. In Sect. 3 the CMDs of UKS 2 and field are presented, showing that we are dealing with an open cluster, and the parameters are derived. Concluding remarks are given in Sect. 4.