1 Introduction

Galactic open clusters have long been considered unrivalled objects not only to undertake structural and evolutionary studies of our own Galaxy's disk but also to enhance our understanding of stellar formation, structure, and evolution. Galactic open clusters owe this privileged status to the relative ease and precision with which their reddenings, distances, and metallicities can be ascertained (see, e.g., Janes et al. 1988; Twarog et al. 1997). CCD photometric surveys of Galactic open star clusters are currently under way with the aim of obtaining good-quality photometric data not only to enlarge the sample of studied clusters, but also to estimate their fundamental parameters reliably (e.g., Phelps 2001). In some cases, however, the main goal is to explore the star formation history and spatial structure in selected regions (e.g., Moitinho 2001), while in others, the primary aim is to catalogue specific types of stars in selected open clusters (e.g., Kalirai et al. 2001). At the same time, systematic surveys to find new star clusters in selected fields (Dutra & Bica 2001), as well as open cluster remnants (Bica et al. 2001; Pavani et al. 2001) are also being undertaken.

As is commonly accepted, the presence of an apparent concentration of stars in the sky does not necessarily lead to the inevitable conclusion that such concentration constitutes a physical cluster. Any of the following factors, or a possible combination of them, could account for differences in the star density in some region: the presence of a genuine open cluster, an accidental distribution of stars along the line of sight, or the inhomogeneous distribution of interstellar material. In the last few years, several CCD photometric studies of open cluster candidates included in the Lyngå (1987) catalogue have been carried out with the main purpose of determining whether they are genuine physical systems (see, e.g., Piatti et al. 2000a, and references therein). Even though in some cases, the studied objects have been confirmed as genuine physical systems (see, e.g. Piatti et al. 1998,2000b), in some others, there exists evidence that the presumed clusters are not real, but rather random fluctuations of stellar density in a given region (e.g., Carraro & Patat 1995; Piatti & Clariá 2001). In some cases, the physical nature of the open cluster candidates is still highly debatable. A typical example is NGC6994, which Bassino et al. (2000) consider a 2-3 Gyr open cluster, but which Carraro (2000) assumes to be simply a random enhancement of four bright stars above the background level rather than an open cluster. Since star clusters are known to evolve dynamically and stellar depletion effects eventually lead to cluster dissolution, it is probable that some unconfirmed clusters are in fact cluster remnants or fossil remains (de la Fuente Marcos 1998; Bica et al. 2001).

We have recently started a systematic survey that consists in obtaining colour-magnitude diagrams (CMDs) of numerous, mostly unstudied, poorly populated objects described as star clusters in open cluster catalogues (Alter et al. 1970; Lyngå 1987; Lauberts 1982). The primary purpose of this survey is to determine whether they are real physical systems or not. In this paper we present the results obtained for five open cluster candidates projected close to the Galactic centre, namely: Ruprecht 103 or ESO130-SC11 (OCL-876, $\alpha_{1950} = 12^{\rm h} 13^{\rm m} 12^{\rm s}$, $\delta_{1950} = -58^{\rm o} 08\hbox{$^\prime$ }$; $l = 298.28^{\rm o}$, $b = 4.14^{\rm o}$), Ruprecht 124 (OCL-1005, $\alpha_{1950} = 17^{\rm h} 24^{\rm m} 30^{\rm s}$, $\delta_{1950} = -40^{\rm o} 44\hbox{$^\prime$ }$; $l = 348.07^{\rm o}$, $b = -3.32^{\rm o}$), Ruprecht 129 or ESO455-SC40 (OCL-1037, $\alpha_{1950} = 17^{\rm h} 44^{\rm m} 06^{\rm s}$, $\delta_{1950} = -29^{\rm o} 35\hbox{$^\prime$ }$; $l = 359.62^{\rm o}$, $b = -0.66^{\rm o}$), Ruprecht 146 or ESO592-SC4 (OCL-39, $\alpha_{1950} = 18^{\rm h} 49^{\rm m} 36^{\rm s}$, $\delta_{1950} = -21^{\rm o} 11\hbox{$^\prime$ }$; $l = 14.08^{\rm o}$, $b = -9.70^{\rm o}$) and Ruprecht 166 or ESO96-SC11 (OCL-899, $\alpha_{1950} = 13^{\rm h} 22^{\rm m} 42^{\rm s}$, $\delta_{1950} = -63^{\rm o} 09\hbox{$^\prime$ }$; $l = 306.80^{\rm o}$, $b = -0.80^{\rm o}$). These five objects were discovered by Ruprecht (1960), who described them as clusters of Trumpler class IV2p, II3p, IV1p, III2p, and III3p, respectively. The ESO identifications are all from Lauberts (1982). Ruprecht 103 is located in Crux, Ruprecht 124 in Scorpius, Ruprecht 166 in Centaurus, and Ruprecht 129 and 146 in Sagittarius, all projected close to the Galactic centre. Only Ruprecht 166 has some information available for it. Moffat & Vogt (1973) observed three comparatively bright stars in the field of Ruprecht 166 using UBV photolectric photometry and concluded that this is a doubtful cluster in view of the relatively high field star density in the vicinity of the supposed cluster region. In Sect. 2 we present the observations and describe the reduction procedures briefly. We analize in Sect. 3 the possible existence of genuine open clusters within the observed sample, following two different approaches. We discuss the results in Sect. 4, and finally we present the concluding remarks in Sect. 5.