[1] CNR, Ist Fis Cosm, I-20133 Milan, Italy
[2] ESO, D-85748 Garching, Germany
[3] Osservatorio Astron Merate, I-22055 Merate, LC, Italy
[4] Open Univ, Dept Phys & Astron, Milton Keynes MK7 6AA, Bucks, England
[5] Osservatorio Astron Roma, I-00040 Rome, Italy
[6] Max Planck Inst Extraterr Phys, D-85740 Garching, Germany
[7] Observ Astron Nacl, Ensenada, Baja California, Mexico