Variable HST guide stars (I)

We have performed time series analyses on more than 4500 sets of Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) data, obtained from about 3600 guide stars in the period from August 1992 to December 1995. We discuss here a subset of 20 stars that were found to be variable at a high confidence level. This subset contains abl definite variables in our sample for which at least spectral type, B - V, or Stromgren colors, respectively, are available. Our variability survey is characterized by comparably high photometric accuracy and high time resolution. The time base of a data set for a given guide star can range from half an hour to several days and even up to years in some cases. The elimination of scintillation noise in a space-borne experiment and the usage of the identical instrument throughout the survey results in very homogeneous data with a typical photometric precision of about 500 ppm, and with a best case of 50 ppm. Half of the cases discussed here are probably delta Set variables, and two are eclipsing binaries. We have also found four K-type stars with a photometric period of a few hours, which is difficult to interpret as rotation. Currently, several space missions are in preparation, dedicated to high precision photometry with the goal of applying asteroseismic techniques to a significant sample of stars. The HST-FGS data provide an excellent testing ground for modelling the photometric characteristics of such space experiments and for gaining experience in handling the, issues related to the respective data and algorithms.