1. Departamento de Matemática, Universidade do Minho, Campus de Gualtar, 4710 Braga, Portugal
2. Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, U.K.
3. Institute of Cosmology and Gravitation, Mercantile House, University of Portsmouth, Portsmouth PO1 2EG, U.K.