1. Institute for Applied Problems in Mechanics and Mathematics, Naukova 3-b, UA-79060 Lviv, Ukraine
2. Astronomical Observatory of the Jagiellonian University, Orla 171, PL-30-244 Kraków, Poland
3. INAF – Osservatorio Astronomico, Piazza del Parlamento 1, I-90134 Palermo, Italy
4. Universität Potsdam, Institut für Physik and Astronomie, Karl-Liebknecht-Strasse 24/25, D-14476 Potsdam, Germany
5. DESY, Platanenallee 6, D-15738 Zeuthen, Germany
6. Dipartimento di Fisica e Chimica, Universitá di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy