1. Achmad, L., de Jager, C., and Nieuwenhuijzen: 1991, Astron. Astrophys. 250, 445.
2. Aggarwal, K.M., Kingston, A.E., and McDowel, M.R.C.: 1984, Astrophys. J. 278, 874.
3. Berrington, K. A. and Kingston, A. E.: 1987, J. Phys. B 20, 6631.
4. Burgess, A., and Tully, J.A.: 1992, Astron. Astrophys. 254, 436.
5. Calisson, M.: 1986, Uppsala Astronomical Observatory report No 33.