1. Baerentzen, J., Gammelgaard, P., Hilberg, T., Joergensen, K. F., Kristensen, H., Nissen, P. E., and Rudkj�bing, M.: 1967,J. Obs. 50, 83.
2. Beichman, C. A., Jennings, R. E., Emerson, J. P., Baud, B., Harris, S., Rowan-Robinson, M., Aumann, H. H., Gautier, T. N., Gillett, F. C., Habing, H. J., Marsden, P. L., Neugebauer, G., and Young, E.: 1984,Astrophys. J. 278, L45.
3. Churchwell, E. and Walmsley, C. M.: 1973,Astron. Astrophys. 23, 117.
4. Deutschman, W. A., Davis, R. J., and Schild, R. E.: 1976,Astrophys. J. Suppl. Ser. 30, 97.
5. Dorschner, J., Friedemann, C., and G�rtler, J.: 1977a,Astrophys. Space Sci. 46, 357.