1. Abbott, D. C., Bieging, J. H. and Churchwell, E. 1984, Astrophys. J., 280, 671.
2. Bertschinger, E. and Chevalier, R. A. 1985, Astrophys. J., 299, 167.
3. Cassinelli, J. P. and Swank, J. H. 1983, Astrophys. J., 271, 681.
4. Castor, J. I. 1970, “Shock Waves in Population II Variable Stars”, in The Evolution of Population II Stars, ed. A. G. D. Philip, Dudley Observatory Report No. 4, 147.
5. Castor, J. I., Abbott, D. C. and Klein, R. I. 1975, Astrophys. J., 195, 157.