Department of Astrophysics, Cosmology and Fundamental Interactions (COSMO),
Centro Brasileiro de Pesquisas Fisicas (CBPF),
Rua Dr. Xavier Sigaud 150,
Rio de Janeiro,
CEP 22290-180, Brazil.

Tel. number: +55-21-21417377.
Fax number: +55-21-21417266.

E-mail address: mcasals _at_ cbpf _dot_ br.