1. Institute of Astronomy of V. N. Karazin Kharkiv National University , Svobody square 4, 61022 Kharkiv , Ukraine
2. INAF–Osservatorio Astrofisico di Torino , Via Osservatorio 20, Pino Torinese, I-10025 Turin , Italy
3. Department of Astronomy, University of Geneva , Chemin Pegasi 51, 1290 Versoix , Switzerland