1. Department
of Physics, Stockholm University, Alba Nova, SE-106 91 Stockholm, Sweden
2. Department
of Physics and Astronomy, Uppsala University, SE- 751 20 Uppsala, Sweden
3. LUTH, Observatoire de Paris, Place Janssen, 92190 Meudon, France
4. Institute
of Physics, Świętokrzyska Academy, ul. Świętokrzyska
15, PL-25406 Kielce, Poland
5. LERMA and ENS, 24 rue Lohmond, 75232 Paris Cedex-05, France