1. Allen, S. et al., 1991. Mon. Not. R. astr. Soc., in press.
2. Amaud, K., 1987. PhD. thesis, University of Cambridge.
3. Briel, U. et al., 1991. Astr. Astrophys., L10.
4. Cowie, L. Henriksen, M. and Mushotzky, R., 1987. Astrophys. J., 337, 593.
5. David, L., Jones, C. and Forman, W., 1990. Astrophys. J.