[1] INAF, Osservatorio Astron Roma, Via Frascati 33, I-00040 Monte Porzio Catone, RM, Italy
[2] INAF, Osservatorio Astron Padova, Vicolo Osservatorio 5, I-35122 Padua, Italy
[3] INAF, Osservatorio Astrofis Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy
[4] INAF, Osservatorio Astron Palermo, Piazza Parlamento 1, I-90134 Palermo, Italy
[5] INAF, Osservatorio Astrofis Arcetri, Largo E Fermi 5, I-50125 Florence, Italy
[6] Leibniz Inst Astrophys Potsdam, An Sternwarte 16, D-14482 Potsdam, Germany
[7] INAF, Osservatorio Astron Brera, Via E Bianchi 46, I-23807 Merate, LC, Italy
[8] INAF, Osservatorio Astron Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
[9] INAF, Ist Astrofis & Planetol Spaziali, Via Fosso del Cavaliere 100, I-00133 Rome, Italy
[10] INAF, Osservatorio Astrofis Catania, Via S Sofia 78, I-95123 Catania, Italy
[11] Univ Padua, Dipartimento Fis & Astron, Vicolo Osservatorio 2, I-35122 Padua, Italy
[12] INAF, Fdn Galileo Galilei, Rambla Jose Ana Fernandez Perez 7, Brena Baja 38712, TF, Spain
[13] INAF, Ist Radioastron, Via Gobetti 101, I-40127 Bologna, Italy
[14] INAF, Osservatorio Astron Trieste, Via Tiepolo 11, I-34143 Trieste, Italy
[15] Wesleyan Univ, Astron Dept, Van Vleck Observ, 96 Foss Hill Dr, Middletown, CT 06459 USA
[16] Univ Roma Tor Vergata, Dipartimento Fis, Via Ric Sci 1, I-00133 Rome, Italy
[17] INAF, Osservatorio Cagliari, Via Sci 5, I-09047 Selargius, CA, Italy