1 Introduction

It has long been recognized that open clusters are extremely important in the study of the structure and evolution of the disc of our Galaxy (Friel 1995; Twarog et al. 1997). Particularly, open clusters located towards the Galactic centre play an important role as they offer the possibility of tracing the structure and evolution of the inner disc. Many open clusters in this Galactic area, however, have not been studied in detail yet because they are affected by high interstellar absorption and/or strong field star contamination. A large number of these unstudied objects are certainly included in the 60% of the approximately 1200 catalogued open clusters which lack a Colour-Magnitude Diagram (CMD) at present (see, http://obswww.unige.ch/webda/).

Thus, we have recently started a systematic survey that consists of obtaining CMDs of numerous, mostly unstudied, open clusters (or candidates), preferably located towards the central parts of the Galaxy (Piatti et al. 1999 and references therein). The main purpose of this long-term programme is to determine whether they are genuine physical systems or if, on the contrary, the apparent enhancement of stars in a sky region is a consequence of random fluctuations of the stellar density in that zone. The CMDs also allow us to determine reddenings, distances and ages of those objects confirmed as open clusters, and therefore, to improve our knowledge of the structure and evolution of the inner Galactic disc, with clusters distributed along a larger baseline in galactocentric distances (Piatti et al. 1995; Piatti 1997).

Open cluster candidates were first selected from the Alter et al. (1970) and Lyngå (1987) open cluster catalogues. Some candidates were then eliminated since they already had associated photometric and/or spectroscopic studies. To facilitate the study through CCD observations, we assigned a higher priority to objects with angular sizes smaller than 4 $\hbox{$^\prime$ }$, thus including in our sample most of the stars concentrated in the regions of interest. In order to avoid saturation or low signal-to-noise ratios, we also took into account the apparent brightness of the clusters. The brightest stars in each object have V $\sim$ 12-14 mag, according to the open cluster catalogues mentioned above. Notice that some objects selected from Alter et al. (1970) could not be real clusters, since many of them were included in the catalogues based on their visual appearance in photographic plates. A visual photographic inspection of these objects reveals, in general, the presence of star condensations which are hard to distinguish from the background. These stellar density differences might be caused by an accidental star distribution along the line of sight, by the presence of irregularly distributed interstellar material, by the presence of a genuine cluster, or by some combination of these three factors. Finally, using the ESO/SRC Sky Atlas J and R plates, we rejected some candidates for any one of the following reasons: they are located near a very bright star, or within H II regions; they are small concentrations very difficult to identify, or they have a small number of sparse stars.

In this paper we present the results obtained for the following selected objects: Melotte 105 (OCI-856, $\alpha_{1950}=11^{\rm h}$17 $.\!\!^{\rm m}$3, $\delta_{1950}=-63$$^\circ$ $14\hbox{$^\prime$ }$; l=292 $.\!\!^\circ$89, b=-2 $.\!\!^\circ$45), Hogg 15 (OCI-891, $\alpha_{1950}=12^{\rm h}$40 $.\!\!^{\rm m}$6, $\delta_{1950}=-62$$^\circ$ $50\hbox{$^\prime$ }$; l=302 $.\!\!^\circ$04, b=-0 $.\!\!^\circ$24), Pismis 21 (OCI-931, $\alpha_{1950}=15^{\rm h}$12 $.\!\!^{\rm m}$8, $\delta_{1950}=-59$$^\circ$ $28\hbox{$^\prime$ }$; l=320 $.\!\!^\circ$36, b=-1 $.\!\!^\circ$79) and Ruprecht 140 (OCI-1, $\alpha_{1950}=18^{\rm h}$18 $.\!\!^{\rm m}$5, $\delta_{1950}=-33$$^\circ$ $12\hbox{$^\prime$ }$; l=0 $.\!\!^\circ$01, b=-8 $.\!\!^\circ$86). The published data on these clusters are rather heterogeneous. Melotte 105 is the only cluster in the sample with several previous studies. UBV photoelectric and photographic observations were carried out by Sher (1965) for a total of 85 stars in the cluster field. More recently, Kjeldsen & Frandsen (1991) performed CCD photometry in the same photometric system for 127 stars. The cluster was also target of uvby CCD photometric (Balona & Laney 1995) and integrated spectroscopic (Santos & Bica 1993; Ahumada et al. 2000) studies. Reddening, distance and age values derived from these studies vary with respect to their mean values around 50, 20, and 80%, respectively. Moffat (1974) also derived fundamental parameters of Hogg 15 using UBV photoelectric photometry of 23 stars located in the cluster field, whereas Orsatti et al. (1998) concluded that some "intracluster'' dust exists in association with Hogg 15, based on UBVRI polarimetric observations of these 23 stars. Moffat & FitzGerald (1977) reported UBV photometry of 11 stars in the field of Pismis 21, one of them being the F8I supergiant HD 135159. Their photometry indicates that HD 135159 is surrounded by at least six early B stars with variable extinction: $<E(B-V)>~= 1.66 \pm 0.13$ (s.d.), which are located at a distance of $\sim$1.7 kpc. Ruprecht 140, as far as we know, has not been studied.

This paper is structured as follows: in Sect. 2 the observations are presented and the reduction procedure is briefly described. The analysis of the cluster CMDs is described in Sect. 3, while the cluster parameters are derived in Sect. 4. Finally, the conclusions of this work are given in Sect. 5.

Figure 1: Schematic finding charts for the fields of Melotte 105 (top-left), Hogg 15 (top-right), Pismis 21 (bottom-left), and Ruprecht 140 (bottom-right). North is up and East is to the left. The sizes of the plotting symbols are proportional to the brightness of the stars