1. Appenzeller, I. and Wolf, B.: 1977,Astron. Astrophys. 54, 713.
2. Herbig, G. H.: 1977,214, 755.
3. Hoyle, F. and Wickramasinghe, N. C.: 1967,Nature 214, 969.
4. Hulst, H. C. van de: 1957,Light Scattering by Small Particles, J. Wiley & Sons, Inc., New York and Chapman & Hall, Ltd., London.
5. Rydgren, A. E.: 1977,Publ. Astron. Soc. Pacific 89, 557.