Optical spectrum of HDE 245770 and an X-ray flare of the transient source A0535+26.

Spectrograms of the Be star HDE 245770, the optical counterpart of the transient X-ray source AO535 + 26, are presented. The spectrograms were obtained over a three-year period using the stellar spectrograph attached to the 6-meter telescope at the Special Astrophysical Observatory in Nizhni Arkhyz, USSR. The dispersion of the spectrograph was 9, 14, and 28 A/mm. A comprehensive list of the lines observed in the range 3450-7000 A is given, and measurements of the principal stellar parameters are provided in a table. Evidence of variability in the spectrum of HDE 245770 is discussed, with emphasis given to an abrupt change occurring around January 1981.