1. U. Finkenzeller and R. Mundt, Astron. Astrophys. Suppl. Ser. 55, 109 (1984).
2. U. Finkenzeller and I. Jankovich, Astron. Astrophys. Suppl. Ser. 57, 285 (1984).
3. C. Friedeman, H.G. Riemann, J. Gurtler, and V. Toth, Astron. Astrophys. 277, 184 (1993).
4. G. A. Galazutdinov, Preprint SAO No. 92 (Spets. Astrofiz. Observ., Nizh. Arkhyz, 1992).
5. A. P. Goodson, K.-H. Bohm, and R. M. Wingle, Astrophys. J. 524, 142 (1999).