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\IBVShead{4357}{31 July 1996} 

\title{GREAT OPTICAL OUTBURST OF A0535+26=V725 TAURI}

\begintext

The recurrent transient X-ray source A0535+26 (HDE\,245770, identical to
V725 Tau optical variable,  was first detected by the satellite Ariel V
(Rosenberg et al., 1975). This Be and neutron star binary system is
actively observed in frequency range from X-ray to infrared (see, for
example, Giovannelli \& Graziati, 1992; Gnedin et al., 1988, Motch et al.,
1991). Photometric behaviour of HDE\,245770 was traced in detail in the
paper by Lyutiy et al., 1989). Hao et al. (1986) investigated the semiregular
secular variation of brightness of the system by using Fourier analysis. 
They noted also some evidence of low amplitude (less than 0\fm 01) 
variation of the brightness on hours' time scale. Besides observers mentioned 
sudden short-time rises in brightness (the so called ``blue flares'') with
large ($\Delta$V = 0\fm 15) amplitudes: R\"ossiger (1978) in
December 1977, Gnedin et al. (1988) in April 1983, Maslennikov (1986)
in April 1985, Berdnik et al. (1990) in November 1986.
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\centerline{\epsffile{4357-f1.ps}}
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Figure 1. The data of dual-channel observations on 28/29 Oct. 1995 in
Johnson's B filter: 
a - the counts of A0535+26 in channel 1 (comparison star counts obtained for
the channel reduction, are seen at UT=23.2 and 24.7 hours);
b - the count of the comparison star in channel 2, reduced to the sensitivity
level of channel 1;
c - V$-$C data in relative magnitudes
\large
\vfill \eject

\centerline{2}

\epsfysize=7cm
\centerline{\epsffile{4357-f2.ps}}
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Figure 2. Observations in channel 1 which demonstrate stability of the
photometer's work (a - during the outburst; b - after the outburst). The sky
background counts are seen below, and the comparison star's count above. The
points are linked by a connecting line
\large

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\centerline{\epsffile{4357-f3.ps}}
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\centerline{Figure 3. An enlarged part of Figure 1c between two of the brightest
individual pulses}
\large
\vskip 5mm

Our observations in the nights 27/28 (V filter), 28/29 October (B filter)
and 20/21 November 1995 (B filter) were the part of the Program  ``Monitoring
of Unique Astrophysical Objects'' of the Russian Ministry of Science.
The 2-3 hours' observational runs were done with dual channel photometer
attached to 0.8 m Ritchey-Chretien telescope at the Mt. Dushak-Erekdag
observational station of Odessa Astronomical Observatory (Dorokhov et al.,
1985). The integration time was 10 seconds. HD\,37170 (m=8.4 mag, A2) was
used as a comparison star. Then the data were reduced to 1 sec. integration time.

The great outburst was recorded in the night 28/29 Oct. 1995 (JD\,2450019)
from UT=22.90 to 24.61 hours. The date of the outburst is close to periastron
passage of the pulsar within the error limits if the orbital parameters by
Margoni et al. (1988) are adopted: T$_0$=2443056 $\pm$ 3 days; P=55.73 $\pm$ 
0.31 days. But when we used more recent orbital solution (Finger et al., 
1994) derived from X-ray observations, such a coincidence was not revealed.
\vfill \eject

\centerline{3}
\vskip 2mm

The data of two channel mode are presented in Figure 1. The observations
started after the outburst had begun. The outburst consists of a great number 
of transient individual pulses. The intensity at the burst's peak was about
five times as large as that on the bottom level this night ($\Delta$m=1\fm 73).

In Figure 2 the stability of the photometer's primary channel work is shown in
two parts of the observational run. The detailed character of the outburst
is better seen in Figure 3 which is an enlarged part of Figure 1. Duration
of one fast brightening amounts to 10-20 sec. only which is comparable
to the integration time. The bottom brightness level between the 
pulses was stable during the outburst. The observed ``undisturbed''
magnitude difference variable--comparison was 1.49 mag in 28 Oct. 1995, the
system became brighter by 0.1 mag in 20 Nov. 1995 in filter B.
\vskip 1cm


\beginauthors
\authorsrightcol
N.I. DOROKHOV
T.N. DOROKHOVA
Astronomical Observatory
Odessa State University
Shevchenko Park
Odessa 270014
Ukraine
\endauthors

\references

Berdnik, E.V., Gorshanov, D.L., Maslennikov, K.L., Somsikov, V.V., 1990,
{\it Pis'ma Astron. Zh.}, {\bf 16}, 1099

Dorokhov, N.I., Dorokhova, T.N., Komarov, N.S., Mkrtichian, D.E., 
Mukhamednazarov, S., 1995, {\it Astron. and Astrophys. Trans.}, {\bf 8},
263

Giovannelli, F., Graziati, L.S., 1992, {\it Space Sci. Rev.}, {\bf 59}, 1

Gnedin Yu.N., Zaitseva, G.V., Larionov, V.M., Lyutiy, V.M., Khozov, G.V.,
Sheffer, E.K., 1988, {\it Astron. Zh.}, {\bf 65}, 1196

Hao, J.-X., Huang, L., Guo, Z.H., 1996, {\it A\&A}, {\bf 308}, 499

Finger, M.H., Wilson, R.B., Hagedon, K.S., 1994, {\it IAU Circ.}, No.\,5931

Lyutiy, V.M., Zajtseva, G.V., Latysheva, I.D., 1989, {\it Pis'ma Astron. Zh.},
{\bf 15}, 421

Margoni, K., Ciatti, F., Mammano, A., Vittone, A., 1988, {\it A\&A}, {\bf 195}, 148

Maslennikov, K.L., 1986, {\it Pis'ma Astron. Zh.}, {\bf 12}, 458

Motch, C., Stella, L., Janot-Pacheco, E., Mouchet, M., 1991, {\it ApJ}, 
{\bf 369}, 490

Rosenberg, F.D., Eiles, C.J., Skinner, G.K., Wilmore, A.P., 1975, {\it Nature},
{\bf 256}, 628

R\"ossiger, S., 1978, {\it IBVS}, No.\,1395

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