[1] Argelander Inst Astron, Hugel 71, D-53121 Bonn, Germany
[2] Univ Barcelona, IEEC, Inst Ciencies Cosmos ICCUB, Marti Franques 1, E-08028 Barcelona, Spain
[3] Johns Hopkins Univ, Dept Phys & Astron, 3400 N Charles St, Baltimore, MD 21218 USA
[4] Univ Padua, Dipartimento Fis & Astron G Galilei, Via Marzolo 8, I-35131 Padua, Italy
[5] Ist Nazl Fis Nucl, Sez Padova, Via Marzolo 8, I-35131 Padua, Italy
[6] INAF, Osservatorio Astron Padova, Vicolo Osservatorio 5, I-35122 Padua, Italy
[7] Ist Nazl Fis Nucl, Gran Sasso Sci Inst, Viale F Crispi 7, I-67100 Laquila, Italy