[1] Univ Ferrara, Dipartimento Fis, I-44122 Ferrara, Italy
[2] Ist Astrofis Spaziale & Fis Cosm, INAF, I-90146 Palermo, Italy
[3] Univ Tubingen, Inst Astron & Astrophys, D-72076 Tubingen, Germany
[4] Osservatorio Astron Cagliari, INAF, I-09012 Capoterra, Italy
[5] Penn State Univ, Dept Astron & Astrophys, University Pk, PA 16802 USA