1. Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
2. Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil
3. Departamento de Física, Centro de Ciencias Naturales e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
4. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utretch, the Netherlands
5. Department of Astrophysics/IMAPP, Radboud University, Nijmegen, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands
6. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
7. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA