1. Allen, M. and Robinson, G.W. 1975, Ap. J., 195, 81.
2. Barsuhn, J. and Walmsley, C.M. 1977, Astr. Ap., 54, 345.
3. Black, J.H. and Dalgarno, A. 1973a, Ap. J. (Letters), 184, L101.
4. Black, J.H. and Dalgarno, A. 1973b, Astrophys. Letters, 15, 79.
5. Black, J.H. and Dalgarno, A. 1977, Ap. J. (Suppl.), 34, 405.