1. Dipartimento di Fisica & Astronomia, Università degli Studi di Bologna, Viale Berti Pichat, 6/2, I - 40127 Bologna, Italy
2. Astrophysics Research Institute, Liverpool John Moores University, IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
3. GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, 92125 Meudon Cedex, France
4. European Southern Observatory, Casilla 19001, Santiago, Chile
5. Universidad de Concepcion, Casilla 160-C, Concepcion, Chile