1. Allen, D.A., Harvey, P.M. & Swings, J.P. 1972, Astr. Astrophys., 20, 333.
2. Allen, D.A., Hyland, A.R., Longmore, A.J., Caswell, J.L., Goss, W.M., & Haynes, R.F. 1977, Astrophys. J., 217, 108.
3. Allen, D.A. & Porter, F.C. 1973, Astr. Astrophys., 22, 159.
4. Altenhoff, W.J., Braes, L.L.E., Olnon, F.M. & Wendker, H.J. 1976, Astr. Astrophys., 46, 11.
5. Balick, B. 1972, Astrophys. Lett., 12, 21.