1. Dipartimento di Fisica, Universitá degli Studi di Milano, Via Giovanni Celoria 16, I-20133 Milano, Italy
2. INAF – Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy
3. Leiden Observatory, Leiden University, PO Box 9531, NL-2300 RA Leiden, the Netherlands
4. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany