One hundred years of observations of the Be star HDE 245770 (the X-ray binary A0535+26/V725 Tau): The end of an active phase?

UBV observations of the X-ray binary system A0535+26/V725 Tau at the Crimea n Station of the Sternberg Astronomical Institute in 1980-1998 are presente d. Based on our and published data, we analyze the photometric history of t he star from 1898. Over a period of 100 years, the star apparently showed a ll three activity phases (B, Be, Be-shell) of Be stars. We conclude that th e X-ray activity of the object is attributable to the 1970-1997 outburst of the Be star due to envelope ejection. The star's colors during the minimum light of 1998 and its 1953-1956 colors (before the outburst) correspond to the spectral type B0-B1 at the color excesses EB-V = 0.74 and EU-B = 0.48, in agreement with the current spectral type O9.7. The minimum light of 199 8 and the color excesses are used to determine the colors of the additional radiation, analyze their evolution during the 1973-1997 outburst, and refi ne the distance to the object (3 kpc). The colors of the additional radiati on at maximum light of the star (1973-1980) match the colors of a hydrogen plasma with T-e = 1.5 x 10(4) K which is optically thick in the Balmer cont inuum. The brightness decline corresponds to a decrease in the optical dept h of the plasma; at V similar or equal to 9.(m)1, it becomes optically thin in the Balmer continuum with T-e = 10(4) K and N-e = 10(10)-10(12) cm(-3). This conclusion is consistent with the model of a circumstellar envelope b ut is inconsistent with the existence of an accretion disk around the neutr on star. All the additional radiation responsible for the optical variabili ty is produced by a single source. The intensity of the H alpha emission li ne at maximum light (1975-1980) is triple its intensity in 1987-1997, when quasi-periodic light fluctuations with P approximate to 1400(d) were observ ed [1]. At this time, the line intensity correlated with brightness. The H alpha line was in absorption at the minimum of 1998, and, at present, the s tar's active phase appears to have ended. (C) 2000 MAIK "Nauka/Interperiodi ca".