COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2997 Konkoly Observatory Budapest 13 March 1987 HU ISSN 0374 - 0676 PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR EV Lac IN 1985 Photoelectric monitoring of the flare star EV Lac has been carried out at the Stephanion Observatory during the year 1985 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 colour of the international UBV system. The telescope and photometer used have been described elsewhere (Mavridis et al., 1982). The transformation of our instrumental ubv system to the international UBV system is given by the following equations: a) For the period June 20-July 10, 1985 V = v_0 + 0.086 (b-v)_0 + 2.160, B-V = 0.376 + 1.035 (b-v)_0. b) For the period August 7-14, 1985 V = v_0 + 0.037 (b-v)_0 + 2.020, B-V = 0.393+ 1.050 (b-v)_0. c) For the period September 2-8, 1985 V = v_0 - 0.020 (b-v)_0 + 1.953, B-V = 0.451+ 1.007 (b-v)_0. The monitoring intervals in UT as well as the total monitoring time for each night are given in Table I. Any interruption of more than one minute has been noted. During the 31.82 hours of monitoring time one flare was observed, the characteristics of which are given in Table II. In this table following characteristics (Andrews et al. 1969) are given: a) the date and universal time. of flare maximum, b) the duration before and after the 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 the instrumental system, f) the standard deviation of random noise fluctuation sigma(mag) = 2.5 log (I_0+sigma)/I_0 during the quiet - state phase immediately preceding the beginning of the flare and g) the air mass at flare maximum. The light curve of the observed flare in the b colour is shown in Fig. 1. Table I Monitoring Intervals Date Monitoring intervals (U.T.) Total Monitoring Time 1985 June 21-22 23h42m-00h13m,00h17m-01h00m, 1h14m 23-24 23 33 -00 04, 00 07 -00 37, 00 39 -01 06. 1 28 24-25 23 30 -00 02, 00 05 -00 32, 00 45 -01 10. 1 24 26-27 23 51 -00 24, 00 27 -01 00. 1 06 27-28 23 22 -23 37, 23 39 -23 50, 23 53 -00 26, 00 38 -01 00. 1 21 29 00 01 -00 31, 00 33 -01 01. 58 July 7-8 22 51 -23 20, 23 23 -23 59, 00 14 -00 45, 00 48 -01 12. 2 00 8-9 22 51 -23 25, 23 28 -00 02, 00 15 -00 46, 00 49 -01 12. 2 02 9-10 23 04 -23 36, 23 38 -00 13, 00 33 -01 06. 1 40 August 7 22 37 -23 01, 23 03 -23 30, 23 35 -23 44. 1 00 8 23 04 -23 20. 16 9-10 22 01 -22 29, 22 32 -22 46, 22 48 -23 02, 23 05 -23 19, 23 21 -23 41, 23 43 -23 57, 00 57 -01 08. 1 55 10-11 22 01 -22 20, 22 22 -22 41, 22 43 -23 05, 23 07 -23 23, 23 25 -23 41, 23 43 -00 02. 1 51 11-12 22 23 -22 47, 22 49 -23 06, 23 11 -23 41, 23 44 -00 14, 00 17 -00 50, 00 52 -01 19. 2 41 14 22 14 -22 28, 22 30 -22 57. 41 September 2 21 36 -22 10, 22 15 -22 54, 22 58 -23 47. 2 2 3-4 22 20 -22 45, 22 50 -23 41, 00 06 -01 03. 2 3 4-5 22 45 -22 59, 23 45 -00 22, 00 29 -01 11. 1 33 6-7 22 24 -22 58, 23 04 -23 46, 00 14 -01 00. 2 2 7-8 22 02 -22 37, 22 43 -23 36, 00 05 -00 35, 00 38 -01 02. 2 22 TOTAL 31h49m Table II Characteristics of the Flare Observed Flare Date U.T t_b t_a Duration No. (max) (min) (min) (min) 1985 August 1 9 23h34m.24 1.10 1.16 2.26 (I_f-I_0)/I_0 P Delta m sigma Air (max) (min) (mag) (mag) mass 0.30 0.30 0.29 0.03 1.019 [FIGURE 1] S. AVGOLOUPIS L. N. MAVRIDIS Department of Astronomy Department of Geodetic Astronomy University of Thessaloniki University of Thessaloniki P. VARVOGLIS Department of Mathematics University of Thrace References: Andrews, A.D., Chugainov, P.F., Gershberg, R.E. and Oskanian, V.S., 1969, Inf. Bull. Var. Stars No. 326 Mavridis, L.N., Asteriadis, G.A. and Mahmoud, F.M., 1982, in: Compendium in Astronomy (ed. by E.G. Mariolopoulos, P.S. Theocaris and L.N. Mavridis), D. Reidel Publishing Company, Dordrecht, Holland, p. 253 [BIBCODE 1982coas.conf..253M ]