COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2128 Konkoly Observatory Budapest 1982 April 26 HU ISSN 0374-0676 PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR EV Lac IN 1981 Photoelectric observations of flare stars have been continued at the National Astronomical Observatory of the Bulgarian Academy of Sciences and the Stephanion Observatory, Greece. In this paper we report about our joint observations of the flare star EV Lac during the summer of 1981. The equipment used at the National Astronomical Observatory consists of a 60cm Cassegrain reflector and a new one-channel UBV photometer, built at the Bulgarian Academy of Sciences. A photoncounting system with an integration time of 1 sec was used. Details of this equipment will be published elsewhere. Here we give only the transformation: of the. instrumental ubv system to the international UBV system for the period under consideration: Delta V = Delta V + 0.10 Delta(b-v), Delta(B-V) = 1.11 Delta(b-v) , Delta(U-B) = 0.80 Delta(u-b) , At the Stephanion Observatory the observations were carried out with the 30-inch Cassegrain reflector of the Department of Geodetic Astronomy, University of Thessaloniki and a Johnson dual channel photoelectric photometer. The transformation of the Stephanion ubv system to the international UBV system for the period under consideration is given by the equations; V = v_0 + 0.011(b-v)_0 + 3.191, B-V = 0.369 + 1.022 (b-v)_0 , U-B = -1.858 + 0.962(u-b)_0 , Table I contains, for each night, the monitoring intervals in UT, the colour in which the observations were made, as well as the total monitoring time. Designation NAO or Steph. O. stands for the National Astronomical Observatory or Stephanion Observatory, respectively. Table I Flare star EV Lac,1981 --------------------------------------------------------------------------- Date Monitoring intervals(U.T.) Total monit. Colour NAO/Steph.O. 1981 time --------------------------------------------------------------------------- June 29/30 23h40m08s-00h02m15s, 000545-003630,004130-005500, 005615-012100. 1h31m07s B Steph.0. 30/1 222845-230800,231045-002630, 002930-012445. 25015 B Steph.0. July 1/2 225935-232835,233005-013635. 23530 B Steph.0. 2/3 221320-232535,232700-014020. 32535 B Steph.0. August 8/9 225140-235317,000839-001511, 001803-011305,013241-015602. 22632 U NAO 9/10 215544-005649,011647-015757. 34215 U NAO 21/22 221420-015629. 34209 U NAO 23/24 223800-011239. 23439 U NAO September 3 202259-212415. 10114 B NAO 5/6 221329-005738. 24409 B NAO 28 192128-232824. 40656 U NAO ---------------- Total 30h40m21s Table II Characteristics of the flares observed Flare Date U.T t_b t_a Duration I_f/I_0 Deltam sigma P Air No 1981 min min min max mag mag min mass August 1 8 23h19m545 0.57 1.1 1.7 1.63 0.53 0.03 0.55 1.008 2 8 23 27 48 0.18 9.2 9.4 12.71 2.76 0.03 7.7 1.006 3 9 00 09 59 0.48 2 2.5 1.41 0.37 0.03 0.4 1.002 4 9 00 22 36 1.43 4.4 5.8 3.16 1.25 0.03 2.4 1.004 5 9 23 28 22 3.03 6.6 9.6 2.11 0.81 0.03 3.1 1.004 6 24 00 56 20 0.25 10.7 11 7.45 2.18 0.03 10.2 1.078 September 7 28 19 25 10 0.25 0.8 1.1 1.38 0.35 0.03 0.2 1.025 During the total of 30h40m monitoring time 7 flares were observed (all of them in "u" colour), the characteristics of which are given in Table II. For each flare the following characteristics (Andrews et al., 1969) are given: a. the date and universal time of maximum, b. the duration before and after maximum (t_b and t_a respectively), c. the total duration of the flare, d. the value of the ratio I_f/I_0 corresponding to flare maximum, where I_f is the total intensity of the star plus flare less sky background and I_0 is the quiet state intensity less sky background, e. the increase of the star magnitude at flare maximum: Delta m(u) = 2.5 log I_f/I_0, where "u" is the ultraviolet instrumental magnitude at NAO, f. the standard deviation of random noise fluctuations in mag. sigma(mag) = 2.5 log(I_0+sigma/I_0), g. the integrated intensity of the flare over its total duration: P = Integral (I_f - I_0) / I_0 dt, h. the air mass. The light curves of the observed flares in colour "u" are shown in Figs. 1-7. We would like to draw attention to the remarkable activity of EV Lac during the night of 8/9 August 1981 with 4 flares observed within 1h10m. [FIGURE 1] [FIGURE 2] Acknowledgements The observations reported in this paper are part of the joint research project under the title "Study of variable stars" carried out by the Department of Astronomy with National Astronomical Observatory, Bulgarian Academy of Sciences, and the Department of Geodetic Astronomy, University of Thessaloniki, Greece. This project is part of the Program for Scientific and Technical Co-operation between Bulgaria and Greece. The authors would like to express their gratitude to the relevant authorities of the respective countries for their support. K.P. PANOV G. ASTERIADIS I. PAMUKCHIEV L.N. MAVRIDIS P. CHRISTOV Department of Geodetic Astronomy Department of Astronomy with University of Thessaloniki National Astronomical Observatory Thessaloniki, Greece Bulgarian Academy of Sciences Sofia - 1000, 7th November Str. 1 Bulgaria. Reference: Andrews, A.D., Chugainov, P.F., Gershberg, R.E. and Oskanian, V.S.: 1969, I.B.V.S. No. 326