COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 4083 Konkoly Observatory Budapest 13 September 1994 HU ISSN 0374 - 0676 PHOTOMETRY OF THE ECLIPSING BINARY FK ORIONIS The eclipsing binary FK Ori (=41.1934 = HD 240601; m=11.8-13.8pg; Sp:A2) was discovered by Morgenroth (1934). He observed three moments of weakenings and then the variable was classified as an Algol type. The full light curve was obtained by Szafraniec (1974) by only visual method. No brightness decrease was observed at the secondary minimum. According to the GCVS the orbital period of the binary is changing and its ephemeris is Min I=HJD 2445680.512 + 1.947529d x E (1) Our UBVR photometric observations of FK Ori were obtained with the 0.6 m telescope at Mt. Maidanak in 1989/92. The comparison star BD+8d852 (V=7.33m; U-B=-0.27m; B-V=-0.09m; V-R=-0.05m; Sp:B9) and the check star BD+9d627 (V=10.09m; U-B= 0.93m; B-V=1.11m; V-R=0.58m) were chosen. We determined the probable error of a single measurement of the variable to be 0.04m2 in U, 0.009m in B, 0.012m in V and 0.020m in R. 78 points in U and nearly 240 points in each BVR filter were made. We analyzed all published times of minima and calculated the following ephemeris: Min I=HJD 2426988.565 + 1.9474226d x E+(61+/-2)^d x 10^-10 x E^2 (2) +/-0.005 +/-0.0000028 The O-C residuals are listed in Table 1 and shown in Figure 1. The period of FK Ori increases at a rate of 0.00000115d per year. However, the times of minima after JD 2437500 are better described by the ephemeris from GCVS. We noted the residuals (O-C)_1 in the last but one column of the Table. Perhaps, the period change is uneven. The light curves of FK Ori in U, B, V, R are shown in Figure 2. The duration of the primary minimum is 0.14p and the partial eclipse has been observed. The secondary minimum is very shallow and its duration is practically equal to the primary one. We also observed the important details at the secondary minimum in U and B. The brightness of the star increased almost by 0.2m in U and 0.1m in B. These measurements were obtained in 1991. In 1992 most observations were made in BVR and we did not see any anomalies at these phases. In general the observations of the binary showed a wider scatter in 1991 than in any other years. Perhaps, a large hot spot was on the surface of the secondary component in 1991. [FIGURE 1] Figure 1. O-C diagram for FK Ori [FIGURE 2] Figure 2. The light curves of FK Ori Table 1. Times of minima of FK Ori Observer Type HJD O-C E (O-C)1 E1 2400000+ Morgenroth (1934) pg 26988.576 +0d011 0 27360.525 +0.002 191 27479.292 -0.024 252 Piotrowski (1935) v 27773.390 +0.013 403 Szafraniec (1974) v 33744.30 +0.05: 3469 34661.511 +0.006 3940 36630.394 -0.010 4951 GCVS pg 45680.512 +0.023 9598 0.000d 0 Manek (1992) v 47207.363 -0.001 10382 -0.011 784 Dedoch (1992) v 47207.380 +0.016 10382 +0.005 784 Borovichka (1992) v 47542.345 +0.002 10554 -0.005 956 present paper pe 48541.430 -0.008 11067 -0.002 1469 48915.356 -0.013 11259 -0.002 1661 48917.304 -0.013 12260 -0.001 1662 Table 2. Photometric values of FK Ori V U-B B-V V-R Max 11.15 +0.47 0.13 0.23 Min I 12.50 0.50 0.24 0.37 Min II 11.24 0.50 0.12 0.14 In Table 2 we list the average photometric characteristics of the curves. Mamnun ZAKIROV Ulugh Beg Astronomical Institute, Astronomicheskaya st. 33, Tashkent, 700052 Uzbekistan References: Borovichka, J., 1992, Brno Obs. and Plan. Contr., 30, 19 Dedoch, A., 1992, ibid. Manek, J., 1992, ibid. Morgenroth, O., 1934, Astron. Nachr., 252, 389 [BIBCODE 1934AN....252..389M ] Piotrowski, S., 1935, Acta Astron. Ser. C., 2, 63, 79 Szafraniec, R., 1974, Acta Astron, 24, 89 [BIBCODE 1974AcA....24...89S ]