COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       NUMBER 669

                                         Konkoly Observatory
                                         Budapest
                                         1972 May 1


      PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR EV LAC


Continous photoelectric monitoring of the flare star EV Lac has been carried
out at the Stephanion Astronomical Station (alpha=-22deg45'44" phi=+37deg45'15")
during the period of cooperative optical observations of this star proposed
by the IAU Working Group on Flare Stars i.e. September 11-27, 1971 (Andrews
et.al. 1971) using the 30-inch Cassegrain reflector of the Department of
Geodetic Astronomy, University of Thessaloniki. Observations have been made
with a Johnson dual channel photoelectric photometer in the B color of the
international UBV system. The telescope and photometer will be described
elsewhere. Here we mention only that the transformation of our instrumental
ubv system to the international UBV system is given by the following
equations:

V=v+2.094+0.022(b-v),
(B-V)=0.837+1.005(b-v),
(U-B)=-1.179+0.993(u-b),

The monitoring intervals in UT as well as the total monitoring time for each
night are given in Table 1. Any interruption of more than one minute has
been noted.

During the 5.9 hours of monitoring time 2 flares were observed the
characteristics of which are given in Table 2. For each flare following
characteristics (Andrews et.al. 1969) are given: a) the date and universal time
of flare maximum, b) the duration before and after maximum (t_b and t_a
respectively) as well as the total duration of the flare, c) the value of the
ratio (I_f-I_0)/I_0 corresponding to flare maximum, where I_0 is the intensity
deflection less sky background of the quiet star and I_f is the total
intensity deflection less sky background of the star plus flare, d) the
integrated intensity of the flare over its total duration, including
pre-flares, if present, P=Integral(I_f-I_0)/I_0 dt, e) the increase of the 
apparent
magnitude of the star at flare maximum delta m(b)=2.5 log(I_f/I_0), where b is
the blue magnitude of the star in our instrumental system, f the standard
deviation of random noise fluctuation sigma(mag)=2.5 log(I_0+sigma)/I_0 and g)
the air mass. The light curves of the observed flares in the b color are shown
in Figs. 1-2.

                            [FIGURE 1-2]

Table 1

Date    Monitoring intervals (UT)                               Total
1971                                                          monitoring
Sep.                                                            time
26-27   19^h49^m-20^h25^m, 2026-2048, 2058-2118, 2119-2200,
        2208-2228, 2230-2250, 2251-2312, 2320-2341,
        2342-2400, 0000-0023, 0032-0052, 0053-0113,
        0114-0133                                              5^h 6^m
27      1852-1912, 1913-1934, 1935-1942                           48^m
                                                      Total    5^h54^m

Table 2

Characteristics of the flares observed

Flare  Date U.T.        t_b   t_a   Durat. I_f-I_0/I_0    P    delta m  sigma  Air
No.    1971 max.        min.  min.                       min.   mag.     mag.  mass
       Sep.                         min.      max.
1.    26 20^h21.^m3     1.6    ?     ?       0.08         ?    0.09    0.011   1.03
2     26 23  34.8       0.9   5.5   6.4      0.10       0.30   0.10    0.014   1.09


Department of Geodetic Astronomy
University of Thessaloniki, Greece
April 18, 1972

                                                  M.E. CONTADAKIS
                                                  L.N. MAVRIDIS

References:

Andrews, A.D., Chugainov, P.F., Gershberg, R.E., and Oskanian,V.S.: 1969,
   Comm. 27 IAU, Inf. Bull. Var. Stars No. 326.
Andrew, A.D., Chugainov, P.F., Kellog, E.M., and Oskanian V.S.: Comm. 27 IAU,
   Inf. Bull. Var. Stars No. 516.