1. Department of Physics, University of Warwick, CV4 7AL Coventry, UK
2. Dr. Karl Remeis-Observatory, Friedrich-Alexander University Erlangen-Nuremberg, Sternwart-str 7, D-96049 Bamberg, Germany
3. Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, D-69120 Heidelberg, Germany
4. Anton Pannekoek Institute for Astronomy, University of Amsterdam, NL-1090 GE Amsterdam, the Netherlands
5. Institute of Physics and Astronomy, University of Potsdam, D-14476 Potsdam, Germany
6. Leicester Institute of Space and Earth Obs., University of Leicester, Leicester LE1 7RH, UK
7. Center for Astrophysical Sciences, Johns Hopkins University, Baltimore, MD 21218, USA