1. Balbinot, R., Brady, P.R., Israel, W. and Poisson, E. (1991) Physics Letters B (submitted)
2. Bardeen, J.M., Carter, B. and Hawking, S.W. (1973) Commun. Math.. Phys. 31, 161
3. Barrabés, C., Israel, W. and Poisson, E. (1990) Class. Quantum Gravity 7, L273
4. Barrabés, C. and Israel, W. (1990) Phys. Rev. D43, 1129
5. Barrabés, C., Israel, W. and Letelier, P.S. (1991) Physics Letters A (submitted)