[1] Univ Roma La Sapienza, Dipartimento Fis G Marconi, I-00185 Rome, Italy
[2] Univ Rome 1, Sez INFN, I-00185 Rome, Italy
[3] Osserv Astrofis Arcetri, I-50125 Florence, Italy
[4] Univ Florence, Dipartimento Astron, I-50125 Florence, Italy
[5] Univ Padua, Dipartimento Fis G Galilei, I-35131 Padua, Italy
[6] Ist Nazl Fis Nucl, Sez Padova, I-35131 Padua, Italy