COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3793 Konkoly Observatory Budapest 26 October 1992 HU ISSN 0374 - 0676 OW Gem: THE 1991 PRIMARY MINIMUM The variability of SAO 095781, which was previously listed as the possible variable NSV 03005, was confirmed by Kaiser et al. (1988a). This star was found to be a long-period eclipsing binary with period 1258.56d by Kaiser (1988b). Its new name is OW Gem. Kaiser predicted the next primary miminum to occur on September 2, 1991. The CCD photometry of OW Gem during the primary minimum presented here is part of an observing program, of which the results will be published soon (Hanzl et al. 1992). From August 30 to September 13, 1991, CCD observations of the long-period eclipsing binary OW Gem were performed at the Ondrejov Observatory during 7 nights. A small SBIG ST-4 camera with 7-cm lens (focal length of 10.5 cm) was used. Having only an 8-bit AD convertor, this camera was not designed as a photometric device, but when certain conditions are fulfilled, it can be used as a multichannel photometer that yields an accuracy of about 0.03 mag. These conditions are the following: * the peak signal of the measured object must be kept in the upper half of the dynamical range of the CCD-camera (that is, 0 to 255), * when the image of the measured object extends over several pixels, the error due to a finite number of levels of analogue-digital (8-bit) conversion is suppressed. These conditions were fulfilled only during the last three nights on account of the strong interference of the Moon during the first four nights. During each observing night, one to three images of the star field around OW Gem - including the variable star and comparison stars - were obtained. The comparison stars were SAO 095777, SAO 095810, and the third star at R. A.(1950) = 6h29m02s, Decl.(1950) = +17d08'05" (Kaiser et al. 1988a). Observing conditions were not ideal; there was strong interference of the Moon before September 5 and measurements were performed at an air mass of about 2.2 at the beginning of dawn. Exposures varied from 15 to 60 seconds, with median value 30 seconds. No filter was used; the maximum of the spectral sensitivity of the camera has a flat maximum around 730 nm with halfwidth of 350 nm (530-880 nm). Dark subtraction and flat fielding of images were performed and also sky subtraction was made. Signals of the variable star and all the comparison stars were measured by integrating counts over a 3x3 pixel area centered at the position of the measured object (such a 3x3 pixel box contains more than 96% of all light of the measured star). In what follows, we briefly describe the reduction procedure. The comparison stars were assumed to be non-variable (checked by residuals after reduction). Co-adding signals from all comparison stars in a particular image we obtained the intensity of a fictitious star less affected by a noise. Differential magnitudes of all comparison stars as well as the variable star with respect to the fictitious star were then obtained. Errors of differential magnitudes of comparison stars were derived from, deviations from the mean values on individual images (errors of summed signals were also accounted for). The accuracy of a differential magnitude of OW Gem at every particular image was estimated from errors of comparison stars by comparing the signal of the variable with signals of comparison stars on the image. Finally, a constant value was added to all differential magnitudes to reduce the mean differential magnitude of SAO 095810 to zero. The mean differential instrumental (ST-4) magnitudes of the comparison stars were SAO 095810: 0.000+/-0.009; SAO 095777: 1.268+/-0.014; the third star: 2.086+/-0.032. The standard errors of differential instrumental magnitudes of OW Gem were found to be between 0.014 and 0.058 (average 0.031 mag), except for measurements on September 2 and 3, when the use of the ST-4 camera as multichannel photometer was not possible. The magnitudes obtained are given in Table 1. Table 1: Differential instrumental (ST-4) magnitudes of OW Gem. UT JD - 2448000 DeltaMag_instr exposure 1992 (seconds) Aug. 30.0936 498.5936 0.698 +/- 0.056 30 Sept. 2.0820 501.5820 1.38 +/- 0.19 30 3.0913 502.5913 1.343 +/- 0.083 30 3.0921 502.5921 1.36 +/- 0.16 15 4.0939 503.5939 1.140 +/- 0.026 30 4.0949 503.5949 1.092 +/- 0.058 15 4.0957 503.5957 1.215 +/- 0.029 60 5.0892 504.5892 0.917 +/- 0.018 30 5.0906 504.5906 0.918 +/- 0.027 60 10.0754 509.5754 0.252 +/- 0.014 30 10.0764 509.5764 0.316 +/- 0.024 15 10.0767 509.5767 0.258 +/- 0.016 60 13.1032 512.6032 0.201 +/- 0.048 30 13.1040 512.6040 0.209 +/- 0.026 30 Our observations confirm the correctness of Kaiser's prediction of the 1991 primary minimum of OW Gem. Combining these measurements with results obtained by D. Hanzl, D. Chochol, and J. Papousek, we derived an improved ephemeris for OW Gem. That analysis will be published by Hanzl et al. (1992). The author is very grateful to the Santa Barbara Instrument Group, Santa Barbara, California, for lending the CCD-camera used in these observations. I also thank Dr. C. Sterken for carefully refereeing and commenting this paper. Petr PRAVEC Astronomical Institute CS-25165 Ondrejov Czechoslovakia References: Hanzl, D., Chochol, D., Papousek, J., Pravec, P., 1992, to be published in Inform. Bull. Var. Stars Kaiser, D.H., Baldwin, M.E., Williams, D.B., 1988a, Inform. Bull. Var. Stars No. 3196 Kaiser, D.H., 1988b, Inform. Bull. Var. Stars No. 3233