1. INAF/Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, RM, Italy
2. Lab. de Physique Mathématique et Théorique, Université Montpellier II, UMR 5825, 34095 Montpellier Cedex 05, France
3. Department of Physics, Gunma National College of Technology, Gunma 371-8530, Japan