COMMISSION 27 OF THE I.A.U.
                INFORMATION BULLETIN ON VARIABLE STARS
                            Number 2954

                                                       Konkoly Observatory
                                                       Budapest
                                                       10 November 1986

                                                       HU ISSN 0374 - 0676


                   FLARE OBSERVATIONS OF EV Lac

This communication presents the results of the observations of the
flare star EV Lac carried out at the National Astronomical Observatory of
the Bulgarian Academy of Sciences. The observations cover the period from
11 to 14 August, 1986, when, together with the reported photoelectric 
observations on the 60 cm telescope and the FF-1 single-channel photoelectric
photometer, more than 40 spectrograms in the blue and red spectral regions
of EV Lac were obtained for a period of two nights. More detailed data
with reference to the character of the spectrum during the photoelectric
monitoring in the U light will be published later.
   The total effective observational time is about 13 hours. The light
curves of the observed flares, as well as some data on the behaviour of
the star's brightness in the light minimum are shown. Table I lists the
respective data on the observed flares: number of flare, date of flare
in U.T., maximum moment in U.T., the duration of the increase (t_1), that
of the decrease (t_2), amplitude in U light (delta m_U) and errors in 
observations (Delta_U)
   It is worth noting that the EV Lac monitoring was carried out excluding 
the optical companion of the star.
   Figure 1 shows the light curves of the observed flares. Universal time
(U.T.) is indicated along the abscissa, while the value (I_f-I_o)/I_o are
plotted along the ordinate where I is the stellar intensity in normal
state and I_f is the stellar intensity with additional emission during the
flare.


                         [FIGURE 1]

   Figure 1: Light curves of observed flares of EV Lac

                         [FIGURE 2]

   Figure 2  Light curves of EV Lac in undisturbed state and of the
             standard star C_2 during the observational period 12 August -
             14 August, 1986.
   

                         Table I
            Data for observed flares of EV Lac

N Date of       Time of       Time of   Time of
flare (UT)    maximum (UT)   increase   decrease   m_U    U

1a 11.08.86    23h38m33s       43s       --     0m52     0m07
1b 11.08.86    22 39 38         8        40s    0.54     0.07
2  12.08.86    01 11 10        15       145     0.52     0.13
3  12.08.86    21 07 10        70       180     0.85     0.06
4a 12.08.86    23 03 40        20        --     0.55     0.06
4b 12.08.86    23 05 00        30       200     0.66     0.06
5  13.08.86    01 24 23         8       240     0.46     0.07
6  13.08.86    01 56 02       1 total           2.44     0.0S
                               duration


                        Table II
          Results of UBV measurements of EV Lac
                and the standard star C2

Date                 EV Lac                   C_2
U.T.            V     B-V    U-B        V     B-V     U-B

12.08.1996
00h15m         10m22  1m33   1m40     10m76   0m19   -0m24
20h25m         10.07  1.28   1.31     10.81   0.18   -0.29
22h32m         10.20  1.23   1.28     10.79   0.19   -0.34


13.08.1986
00h56m         10.22  1.35   1.42     10.79  -0.08   -0.26
22h43m         10.09  1.38   1.32     10.79   0.12   -0.21

14.08.1986
02h00m         10.07  1.40   1.20     10.78   0.22   -0.30.

   We should like to attract particular attention to the case of flare No.6
where a spike-shaped increase of the light of EV Lac was again recorded as
during the 1984 observations (Tsvetkov et al., 1985). The nature of this sort
of flares is still controversial and it is therefore necessary simultaneous
observations in the ultraviolet spectral region to be carried out in order to
prove and understand the physical conditions of its appearance.
We can further say that flare No. 6 is also comparable in character and
amplitude to the phenomenon observed by Gershberg and Petrov (1985) during
simultaneous observations with ASTRON.
   The UBV photometry of EV Lac in the light minimum was carried out by
employing the photometric standards C_1 and C_2 (Pettersen and Jin-Chung Hsu,
1981, Pettersen, 1980). It should be noted here that the star C_2 is of
spectral class A3V. During the processing of the observational data it was
found out that this star exhibits light variations in the U and B colour
with an amplitude of about 0m.3, while in the V light it was constant (Fig. 2).
This result brings to question the use of C_2 as a standard star in 
investigations of the light variations of EV Lac. On the other hand, using the
standard star C_1, relatively short-time light and colour variations of EV Lac
were recorded by Pettersen (1980). Table II is a summary of the results of
the measurements for EV Lac and the standard star C_2.
   Thanks are due to L. Iliev for his assistance in part of the observations.


M.K. TSVETKOV, K.P. TSVETKOVA              N.D. MELIKIAN
 Department of Astronomy and            Byurakan Observatory
National Astronomical Observatory          Armenian SSR
 Bulgarian Academy of Sciences                 USSR
      Sofia, Bulgaria


References:

Gershberg, R.F., Petrov, P.P., 1986, Proc. Symp. "Flare Stars and Related
   Objects", Byurakan, ed. by L.V. Mirzoyan [BIBCODE 1986fsro.conf...38G ]
Pettersen, B.R., 1980, Astron. J. 85.7.871 [BIBCODE 1980AJ.....85..871P ]
Pettersen, B.R., Jin-Chung Hsu, 1981, Astrophys. J. 247.3.1.1013 [BIBCODE 1981ApJ...247.1013P ]
Tsvetkov, M.K., Antov, A.P., Tsvetkova, A.G., 1986, 
   Comm. Konkoly Obs. Hungarian Acad. Sci., Budapest, No. 86.423 [BIBCODE 1986CoKon..86..423T ]
				[CoKon No. 86]