COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 1395

                                     Konkoly Observatory
                                     Budapest
                                     1978 March 21



  OBSERVATIONS OF HDE 245770 DURING THE RECENT X-RAY FLARE-UP
             OF ITS COUNTERPART A0535+26



    By a lucky chance the flare-up of the transient X-ray source
A0535+26 in December 1977 (reported by M. Chartres and F. Li in
IAUC 3154) could, for the first time, also be followed up its 
optical counterpart HDE 245770. There is no longer any doubt about a
physical connection of these objects. Earlier flare-ups of the X-ray
source took place at 1975 April 28 (IAUC 2774) and 1975 November 
7 (IAUC 2863) (both were spells of maximal intensities).
    In 1973 the author discovered the variability of the Bpe
star HDE 245770 (Astron. Nachr. 295, 47). Since then it has 
continuously been observed by photoelectric UBV photometry at the
Sonneberg 60 cm telescope II. From about the time of the recent
X-ray flare-up we have measurements of 3 nights at our disposal.
The object was compared with the star HDE 245906 with the following 
adopted magnitudes (see Mitt.Verand. Sterne 7, 105): V=10.54m,
B-V=+0.45m, U-B=+0.22m. It should be noted that the magnitude of
the object in V at 1977 Dec. 18.9 (UT) was the brightest one known
to the author. In the other colours this maximum is less marked.

       J.D.         1977/78 UT    V      B     U
    2443496.37      Dec. 18.87   8.72m 9.41m 8.93m
        498.59           21.09   8.87  9.45  8.93
        524.46      Jan. 15.96   8.86  9.44  8.88

(standard deviations in V and B about +-0.02 mag, in U about
+-0.05 mag.).
    Independent of the author's observations, the star was also
measured at that time by C. Bartolini et al. and A. Giangrande
et al. Their differences to the comparison stars HD 35170 and
HD 37438 are published in IAUC 3167. The values obtained on the
basis of the magnitude V=5.15m for HD 37438 given by D.L. Crawford
(Astrophys.J. 137, 523) are plotted in the diagram (open circles)
beside the values obtained by the author (filled circles). The
vertical bars in the diagram give the range of the standard deviations;
the arrow marks the spell of maximum X-ray intensity.
   It is remarkable that the star enters maximum brightness about
5 days earlier than the X-ray source. This behaviour will be hard
to explain if the small increase in optical brightness is 
understood as an effect of heating the optical component of a binary
system by a compact X-ray component.
  Besides, HDE 245770 is the brightest optical counterpart of
a transient X-ray source that has ever been detected.

                      [FIGURE 1]


                         S. ROSSIGER
                  Zentralinstitut fur Astrophysik
                      Sternwarte Sonneberg
                       DDR-64 Sonneberg