1. Astronomy Unit, Physics Department, Trieste University, Via Tiepolo 11, I-34143 Trieste, Italy
2. I.N.A.F., Osservatorio Astronomico di Trieste, I-34143 Trieste, Italy
3. Physics Department G. Occhialini, Milano–Bicocca University, Piazza della Scienza 3, I-20126 Milano–Bicocca, Italy