1. Barcons, X. and Fabian, A. C. 1988, MNRAS, 230, 189.
2. Danese, L. et al. 1986, Astr. Ap., 161, 1.
3. Dressier, A. 1988, Ap.J., 329, 519.
4. Goicoechea, L. J. and MartínMirones, J. M. 1989a, Astr. Ap., in press.
5. Goicoechea, L. J. and Martín-Mirones, J. M. 1989b, preprint.