COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2860 Konkoly Observatory Budapest 7 February 1986 HU ISSN 0374 - 0676 ON THE PERIOD OF YY GEMINORUM YY Gem (=Castor C) is an eclipsing binary consisting of two M1 dwarfs. The system was monitored by IUE and EXOSAT on 14/15 November, 1984 (see e.g. Geyer and Kamper 1985). I observed YY Gem on the same night with the 1-m telescope of Konkoly Observatory at Piszkesteto. The comparison star was BD+31deg1627, a star near enough the variable to make the corrections for differential extinction unnecessary. The background measurements were taken about halfway between the variable and comparison. The full moon and the nearness of Castor made the the U observations useless. Table I below shows the Julian date and the magnitude and colour differences (variable-comparison). Table I J.D. V B-V J.D. V B-V 2446019+ 2446019+ .5274 -0.738 0.082 .5758 -0.385 -0.093 .5287 -0.736 0.069 .5770 -0.382 -0.088 .5299 -0.734 0.074 .5783 -0.377 -0.099 .5352 -0.737 0.084 .5835 -0.341 -0.092 .5364 -0.736 0.080 .5847 -0.345 -0.101 .5377 -0.738 0.082 .5860 -0.343 -0.100 .5433 -0.748 0.084 .5919 -0.452 -0.060 .5446 -0.740 0.073 .5932 -0.470 -0.057 .5459 -0.738 0.067 .5944 -0.490 -0.062 .5513 -0.685 0.054 .5997 -0.560 -0.004 .5526 -0.675 0.034 .6010 -0.570 -0.004 .5538 -0.668 0.032 .6022 -0.580 -0.009 .5599 -0.612 0.001 .6075 -0.650 0.022 .5612 -0.594 -0.004 .6088 -0.659 0.022 .5624 -0.586 -0.004 .6101 -0.677 0.038 .5678 -0.508 -0.054 .6154 -0.716 0.020 .5691 -0.485 -0.087 .6167 -0.751 0.026 .5703 -0.468 -0.072 .6180 -0.761 0.021 There is a controversy concerning the period of YY Gem. Leung and Schneider (1978) found a period different from that given in the GCVS. Later, however, Mallama (1980) could not confirm this new period, his time of minimum was in agreement with that predicted by the catalogue elements. Therefore it is interesting to construct the O-C diagram for YY Gem. The times of the primary and secondary minima found in the literature are listed in Table II (the letters p and s after J.D. mean primary and secondary, respectively). Table II J.D. E O-C Reference 2424500.55 p 5527. 0.00684 van Gent (1926) 2424573.42 s 5616.5 0.00586 van Gent (1926) 2424584.41 p 5630 0.00195 van Gent (1926) 2424591.34 s 5638.5 0.00293 van Gent (1926) 2424595.41 s 5643.5 0.00781 van Gent (1931) 2424619.43 p 5673 0.00684 van Gent (1926) 2424639.39 s 5697.5 0.01074 van Gent (1926) 2424791.65 s 5884.5 0.00879 van Gent (1931) 2424848.65 s 5954.5 0.00879 van Gent (1931) 2424875.53 s 5987.5 0.00977 van Gent (1931) 2424916.65 p 6038 0.00879 van Gent (1931) 2424920.31 s 6042.5 0.00879 van Gent (1931) 2424921.53 p 6044 0.00781 van Gent (1931) 2424922.34 p 6045 0.00586 van Gent (1931) 2424961.43 p 6093 0.00781 van Gent (1931) 2425230.55 s 6423.5 0.00781 van Gent (1931) 2425234.62 s 6428.5 0.00586 van Gent (1931) 2425242.36 p 6438 0.00684 van Gent (1931) 2425687.37 s 6984.5 0.00977 van Gent (1931) 2425698.36 p 6998 0.00684 van Gent (1931) 2427158.36 p 8791 0.00684 Binnendijk (1950) 2427160.40 s 8793.5 0.00781 Binnendijk (1950) 2428545.49 s 10494.5 0.00488 Binnendijk (1950) 2428571.55 s 13526.5 0.00879 Binnendijk (1950) 2428596.39 p 10557 0.00488 Binnendijk (1950) 2429639.48 p 11838 0.00586 Binnendijk (1950) 2430466.39 s 12853.5 0.00488 Binnendijk (1950) 2432605.91 p 15481 0.00684 Kron (1952) 2432695.83 p 15923 0.00684 Kron (1952) 2440969.00 s 25751.5 0.00000 Leung and Schneider (1978) 2440969.82 s 25752.5 0.00586 Leung and Schneider (1978) 2440970.63 s 25753.5 0.00781 Leung and Schneider (1978) 2440971.86 p 25755 0.00976 Leung and Schneider (1978) 2442829.63 s 28036.5 0.00195 Mallama et al. (1977) 2443949.68 p 29412 0.00098 Mallama (1980) 2443960.67 s 29425.5 0.00293 Mallama (1980) 2443969.63 s 29436.5 0.00000 Mallama (1980) 2446017.55 s 31951.5 0.00000 Geyer and Kamper (1985) 2446019.58 p 31954 0.00000 present paper [FIGURE 1] The O-C diagram of YY Gem is plotted in the Figure. Full symbols mean primary, the empty ones mean secondary minima. The O-C values were calculated using the elements: 2420000.0 + 0.81428254d*E. The Figure shows that the period of YY Gem slightly differs from that given in the GCVS. Recording to the Figure the period of YY Gem was 0.81428242d between J.D. 2424500 and J.D. 2433000, and 0.81428155d from J.D. 2440968. E. ZSOLDOS Konkoly Observatory Budapest XII, P.B.67 1525 HUNGARY References: Binnendijk, L. 1950, B.A.N. 11, 203 [BIBCODE 1950BAN....11..203B ] Geyer, E. H. and Kamper, B.-C. 1985, IBVS No. 2819 Kron, G. E. 1952, Astrophys. J. 115, 301 [BIBCODE 1952ApJ...115..301K ] Leung, K. C. and Schneider, D. P. 1978, Astron. J. 83, 618 [BIBCODE 1978AJ.....83..618L ] Mallama, A. D. 1980, P.A.S.P. 92, 468 [BIBCODE 1980PASP...92..468M ] Mallama, A. D.: Skillman, D. R.; Pinto, P. A. and Krobusek, B. A. 1977, IBVS No. 1249 van Gent, H. 1926, B.A.N. 3, 121 [BIBCODE 1926BAN.....3..121V ] 1931, B.A.N. 6, 99 [BIBCODE 1931BAN.....6...99V ]