1. Barros, M. 1997. Math. Proc. Camb. Phil. Soc.
121, 321.
2. Barros, M. 1998. Nuclear Physics B
535, 531.
3. Barros, M. 2000. Classical and Quantum Gravity (to appear).
4. Barros, M., Ferrández, A., Lucas, P., and Meroño, M. A. 1999. Trans. AMS. (to appear).
5. Barros, M., Ferrández, A., Lucas, P., and Meroño, M. A. 1995. C.R. Acad. Sci. Paris
321, 505.