1. Center for Nuclear Study, University of Tokyo, Hirosawa, Wako, Saitama 351-0198, Japan
2. Institute for Physics and Astronomy, University of Aarhus DK-8000 Aarhus C, Denmark
3. Center for Astrophysics, University of Notre Dame, Notre Dame, IN 46556, U.S.A.
4. National Astronomical Observatory, Osawa, Mitaka, Tokyo 181-8588, Japan