1. Abbott, D.C.: 1982, Astrophys. J 259, 282
2. Appenzeller, I.: 1986, IAU Symp. 116, Luminous Stars and Associations in Galaxies C. W.H de Loore et al. eds.,p. 139
3. Appenzeller, I., Wolf B., 1981, in “The Most Massive Stars”, S. D’Odorico et al. eds., ESO Proceedings, Garching 1986, p. 131
4. Davidson, K., 1987, Astrophys. J. 317, 760
5. de Jager, C., 1980, The Brightest Stars. D. Reidel, Dordrecht1980