1. Efanov, V., Moiseev, I., and Severny, A., Int. astr. Union Symp. No., 35 (edit. by Kiepenheuer, K. O.), 588 (1968).
2. Severny, A., Q. Jl R. astr. Soc., 12, 363 (1971).
3. Leighton, R., Noyes, R., and Simon, G., Astrophys. J., 135, 471 (1962).
4. Durasova, M., Kobrin, M., Losovsky, J., Chandajev, A., and Judin, O., Astr. Tsirk. Byuro astr. Soobshch., 531, (1969).