COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      NUMBER 874

                                 Konkoly Observatory
                                 Budapest
                                 1974 March 4


     CONTINUOUS PHOTOELECTRIC OBSERVATIONS OF EV Lac
         DURING THE 1973 INTERNATIONAL PATROL



   The flare star EV Lac was observed photoelectrically with
the 30 cm Cassegrain reflector at Oslo Solar Observatory,
(lambda=0h43m02s, phi=+60d12'30", h=585 m) which is operated by the
Institute of Theoretical Astrophysics, University of Oslo. The
observing session lasted August 22 - September 4, 1973, in 
accordance with the program of the IAU Working Group on Flare
Stars (1).
   The monitoring was performed in the B-band as described
in (2), and included the light from both components of the
system. A detailed presentation of the monitoring intervals
is found in Table 1, noting all interruptions exceeding one
minute. The third column contains weighted mean values with
respect to time of sigma/I_0. A total coverage of 21.83 hours 
resulted in 2 observed flares, the physical characteristics of
which are presented in Table 2.
   Values in the instrumental system are denoted with b.
The transformation to the standard Johnson system was performed
with mean transformation and extinction coefficients (3).
The colours needed for this computation are taken from the
three-colour observations of flare no. 2 at September 3, UT 20h23m.
The value of L(B) needed for calculation of W(B)is taken from
(4).
   Flare no. 2 started at UT 20h18m15s, and the rise phase
was measured until amplitude Delta m_b = 0.64 magnitude was reached.
By accident a sky measurement was performed for the next 30 seconds. 
Thus we lost the first maximum, which from our observations
occurred at UT 20h19m05s +/- 13s.
   30 seconds after ending the sky measurement a secondary
maximum appeared. The values in Table 2 refer to this maximum.
10 seconds before UT 20h21m a very slight rise was noticed, and
at UT 20h23m a definite rise set in. This rise is also indicated
by the U measurements performed in 1 minutes intervals from
UT 20h23m10s and lasting 5 - 8 seconds. In the V band measurements
it started at UT 20h23m30s and lasted for 10 - 15 seconds
with intervals of about 1 minute. There are no significant rise
in the V band at the time of the last light increase. Due to U
and V measurements the last rise was not fully monitored.
    At UT 20h42m the star had not yet completely reached its
pre-flare intensity level.
    When computing the energy released in flare no. 2, the
gaps in the light curve were bridged by linear interpolation.
The energy values given are therefore lower limits.
    A reduction to one component may be obtained by using the
relation given by (5).
    The light curves were constructed from 5 seconds means.
The smoothed curves in the instrumental b-colour are presented
in the figures.
    We are indebted to observator R. Brahde for giving us the
opportunity to use the equipment at the observatory.


Oslo, February 18, 1974.


                    B.N. ANDERSEN     B.R. PETTERSEN
                    Institute of Theoretical Astrophysics
                    University of Oslo
                    P.O. Box 1029, Blindern,
                    Oslo 3. Norway.

References:

(1) Chugainov, P.F. I.B.V.S. No. 744, 1972.
(2) Pettersen, B.R., Andersen, B.N., I.B.V.S. No. 791, 1973.
(3) Sivertsen, S. Inst. of Theoretical Astrophys, Blindern-Oslo.
           Report No. 34, 1972.
(4) Gerschberg, R.E. Astrophys. and Space Sci., 19, 75, 1972. [BIBCODE 1972Ap&SS..19...75G ]
(5) Cristaldi, S., Rodono, M. Astr. and Astrophys. Suppl., 2,
           223, 1970. [BIBCODE 1970A&AS....2..223C ]


                         Table 1
                  Coverage of EV Lac

1973   Detailed Coverage (UT)                      (sigma/I_0)  Time
Aug.                                                 
21.    21h40m-22h20m,2237-2255,2258-2339,2348-2400   0.07 111m
22     0001-0018,0020-0030,0112-0123,0138-0147,
       0149-0157.                                    0.08  55
23     2037-2048,2050-2110,2127-2131,2135-2210,
       2212-2344,2351-2400.                          0.07 171
24     0100-0122,0124-0130,0132-0145.                0.09  41
       2103-2117,2119-2311,2314-2326,2338-2400.      0.06 160
25     0001-0031,0046-0140,0142-0200.                0.06 102
       2105-2126,2128-2141,2144-2220,2222-2306,
       2309-2336,2342-2400.                          0.06 159
26     0000-0108,0122-0144,0146-0200.                0.06 104
27     2029-2052,2057-2120,2127-2135,2232-2306,
       2332-2336.                                    0.11  92
28     0024-0033,0045-0052,0103-0119,0135-0212.      0.09  69
Sep.
3      2006-2032,2033-2049,2102-2204,2210-2226,
       2321-2333.                                    0.06 131
4      0016-0020,0033-0037,0042-0103,0108-0205,
       0211-0221,0223-0236,0237-0243.                0.07 115
       Total coverage: 21h50m


       Table 2.
   Physical Characteristics of the Observed Flares
                                 Max. intensity
Flare  Date  t_max  
No.    1973   UT      Duration (min.) (I_0+f -I_0)/I_0 Delta m(B) sigma/I_0(b)   P     Air    W(B)
                      tau_b   tau_a                   (mag)                    (min)   mass (10^32 erg)
1      Aug. 00h21m29s   0.2     5.5    b:0.35                      0.07        b:0.55  1.041
        22                             B:0.40         0.37                     B:0.74          2.2
2      Sept.20 19 50     - ~23         b:1.03                      0.08        b:7.40  1.168
         3                             B:1.38         0.94                     B:10.00         3.0


                          [FIGURE 1]

                          [FIGURE 2]