1. Auman, J.R. and Woodrow, J.E.J. 1975, Astrophys. J. 197, 163.
2. Bell, R.A., Dickens, R.J. and Gustafsson, B. 1979, Astrophys. J. 229, 604.
3. Blackwell, D.E. and Shallis, M.J. 1979, Monthly Notices Roy. Astron. Soc. 186, 673.
4. Carbon, D.F., 1979, Ann. Rev. Astron. Astrophys. 17, 513
5. Dravins, D., Lindegren, L. and Nordlund, Å. 1980, Astron. Astrophys., in press.