1 Introduction

The large amount of dust and gas in the direction of the galactic bulge is known to be responsible for the large extinction which makes very difficult the observation of the bulge in several bands. However, it has long been known that the irregular distribution of dust and gas presents some "holes" where the extinction is considerably lower. The best known of these so-called windows has been found by Baade (1951) and extensively studied since then. Generally speaking, these low-extinction windows offer a unique opportunity to obtain information about the bulge in the optical range. Several groups (MACHO, OGLE, for example) have been recently monitoring the bulge in search of microlensing events (see Paczynski 1996, for a review).

Since April 1997 we have been conducting a small monitoring project of 12 selected low-extinction windows (see Table 1), taken from Blanco & Terndrup (1989) and Blanco (1988), using the recently refurbished Meridian Circle of the Abrahão de Moraes Observatory (see below). The main objective of the project is the discovery and classification of variable stars. The final goal will be to have an on-line, real time processing data to stimulate the study of potentially interesting events by other observing facilities.

Meanwhile, we have faced the problem that the total number of reference stars of the usual catalogues inside the observed low-extinction fields (13' in declination and between 3 and 6 minutes in right ascension) is small. This posed a serious difficulty for the reduction of data and the search for variable objects. As a first step towards a comprenhensive study of the windows, we have attempted to construct dense (secondary) reference catalogues intended to be of general use. The publication of the Tycho Catalog (ESA, 1997) has added an additional motivation to our task, since the refinement of the latter beyond the completeness limit ($V \, \sim \, 11$) was already foreseen as a useful tool for future observations.

We shall describe in the next section the selected fields. Section 3 is devoted to a presentation of the instrumental facilities and data reduction. The criteria of the selection of the set of standards is discussed in Sect. 4. A general discussion in Sect. 5, the light curves in Appendix A and the new catalogues in Appendix B closes the present work.