1. V. L. Afanasiev and A. V. Moiseev, Astronomy Letters 31 (3), 194 (2005).
2. N. Deminova, V. Shimansky, N. Borisov, et al., in Proc. All-Russian Conf. on Ground-Based Astronomy in Russia. 21st Century, Nizhny Arkhyz, Russia, 2020, Ed. by I. I. Romanyuk, I. A. Yakunin, A. F. Valeev, and D. O. Kudryavtsev, pp. 260–266 (IP Reshenilenko P.A., Pyatigorsk, 2020).
3. U. Heber, Publ. Astron. Soc. Pacific 128 (966), 082001 (2016).
4. D. V. Ivanova, N. A. Sakhibullin, and V. V. Shimanskii, Astronomy Reports 46 (5), 390 (2002).
5. H. Ritter, Astron. and Astrophys. 169, 139 (1986).