1 Introduction

The three Fine Guidance Sensors (FGSs) are photometric instruments that allow to point the Hubble Space Telescope (HST) with high precision to almost any point in the sky by locking on to a pair of stars taken from the Guide Star Catalog or GSC (Lasker et al. 1990; Russell et al. 1990; Jenkner et al. 1990). During a typical HST observing sequence, two FGSs measure simultaneously and with high time resolution the brightness of these guide stars. A detailed description of the instruments can be found in Bradley et al. (1991) and in Kuschnig et al. (1997), the latter being an investigation of the suitability of FGS photometric data for micro-variability surveys.

Since the start of the HST mission in April 1990 till the end of 1999, about 22000 stars have been used for guiding. The corresponding FGS data are available in the Engineering Subset Data Files of the Hubble Data Archive. The goal of our project is to perform a time series analysis for all useful data obtained by the FGSs, possibly until the end of the HST operational life time. Each FGS is an assembly of two interferometers with two channels each. The signal of 4 PMT's per FGS instrument was averaged to a single data point for our analysis. Presently, we have investigated about 4500 individual data sets recorded from more than 3600 guide stars.

With a basic integration time of 25 msec the FGS photometry has a high time resolution, but about half of the FGS light curves are shorter than 45 min, corresponding to about half an HST orbit. Fortunately, there are numerous cases where guide stars have been observed continuously for more than a day, or were revisited by HST after months or even years. Prominent examples are the two guide stars used continuously over more than ten days during the northern Hubble Deep Field program (Zwintz et al. 1999).

As the brightness of guide stars ranges from about V=9 to 14.5, spectral types and photometric colors are available from the SIMBAD archive only for the brightest guide stars, or 7% of the entire sample (Fig. 1). The TYCHO catalog (ESA 1997) provided Johnson B-V values for about 20% of our sample. In addition, Strömgren uvby measurements were obtained with the Danish 0.5 m telescope at ESO (La Silla, Chile) for about a hundred guide stars. Figure 1 illustrates the fact that little or no additional information can be retrieved for guide stars fainter than 11$^{\rm th}$ mag from existing archives.

In this paper we present 20 variable stars for which spectral types or TYCHO B-V values are available.

Figure 1: Brightness distribution of total sample of 3692 guide stars analyzed to date (white columns), of guide stars with TYCHO B-V values (grey columns), and of guide stars with published spectral types (black columns)