1. Instituto de Astrofísica de Canarias, s/n, E-38205 La Laguna, Tenerife, Spain
2. Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
3. Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, the Netherlands
4. National Centre for Nuclear Research, Astrophysics Division, PO Box 447, PL-90-950 Łódź, Poland
5. Janusz Gil Institute of Astronomy, University of Zielona Góra, ul. Licealna 9, PL-65-417 Zielona Góra, Poland
6. Dipartamento di Fisica, Universitá degli Studi Roma Tre, Via della Vasca Navale 84, Rome I-00146, Italy
7. INFN Sezione di Roma 3, Via della Vasca Navale 84, Rome I-00146, Italy
8. INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone (RM), Italy