We thank G. Boillot, J.-C. Sibuet, and D. Masson, who were referees of this paper, for their constructive criticisms that helped significantly to improve this work. Thanks also to A.S. Andrade, J. F. Santos, H. A. Mendes (UA) and P. Terrinha (UL), B. Barbosa (IGM), P. Cunha, J. Dinis, and H. Henriques (UC), and S. Srivastava (GSCA) for helpful discussions and comments. Many thanks to E. Martins for her help with the revision of this paper. Many thanks to V. Hervalejo, G. Marques, M. Pessoa (UA), John Taylor (OU) and Rose Edwards (IODSL) for their work on some of the figures.