1. Department of Astronomy, University of Geneva , Chemin d’Ecogia 16, CH-1290 Versoix , Switzerland
2. INAF, Osservatorio Astronomico di Brera , Via E. Bianchi 46, I-23807 Merate , Italy
3. Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, University of Tuebingen , Sand 1, D-72076 Tübingen , Germany