1. Alcock, C. and Paczy?ski, B.: 1968,Astrophys. J. 223, 244.
2. Auer, L. H. and Mihalas, D.: 1972,Astrophys. J. Suppl. 24, 193.
3. Brun, A.: 1935,Publ. Obs. Lyon, Vol.1, No. 12.
4. Chiosi, C., Nasi, E., and Bertelli, G.: 1979,Astron. Astrophys. 74, 62.
5. Churchwell, E., Mezger, P. G., and Huchtmeier, W.: 1974,Astron. Astrophys. 32, 283.