COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1803 Konkoly Observatory Budapest 1980 June 16 PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR YZ CMi IN 1976 Continuous photoelectric monitoring of the flare star YZ CMi has been carried out at the Stephanion Observatory (lambda=-22deg49'44" phi=+37deg45'15") during the year 1976, 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 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_o + 0.042(b-v)_o + 2.278, (B-V)= 0.706 + 1.043(b-v)_o, (U-B)=-2.550+ 1.490(u-b)_o. 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. In the fourth column of Table I the standard deviation of random noise fluctuation sigma(mag)= 2.5 log(I_o+sigma)/I_o for different times (UT) of the corresponding monitoring intervals is given. During the 4.53 hours of the monitoring time one flare was observed the characteristics of which are given in Table II. For this flare 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_o)/I_o corresponding of flare maximum, where I_o is the intensity deflection less sky background of the quiet star and T A B L E I Monitoring Intervals in 1976 Date Monitoring intervals (U.T.) Total Monitoring sigma(U.T) 1976 Time NOV 28 00h54m -01h32m, 01h34m -02h20m, 02h22m -03h13m. 2h15m 0.05(01h23m),0.04(02h09m), 0.04(02 51 ). 30 00 12 -00 56, 00 58 -01 20, 01 27 -01 33 0.05(00 40 ),0.06(01 13 ), 01 36 -01 59, 02 02 -02 15, 02 25 -02 54. 2h17m 0.06(01 52 ),0.05(02 39 ). TOTAL 4h32m T A B L E II Characteristics of the Flare Observed Flare Date U.T. t_b t_a Duration I_f-I_o/I_o P Delta m sigma Air No max min min min max min mag mass 1976 Nov. 1 28 02h11.67m 0.38 2.33 2.71 0.20 0.12 0.20 0.04 1.22 [FIGURE 1] 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_o)/I_o)dt, e) the increase of the apparent magnitude of the star at flare maximum Delta m(b)=2.5 log(I_f/I_o), 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_o+sigma)/I_o during the quietstate 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. Acknowledgement: The support of the National Hellenic Research Foundation for this research is gratefully acknowledged. G. ASTERIADIS, G. KAREKLIDIS, L.N. MAVRIDIS Department of Geodetic Astronom. University of Thessaloniki Reference: Andrews, A.D., Chugainov, P.F., Gershberg, R.I. and Oskanian, V.S.: 1969, I.B.V.S. No. 326