COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3528 Konkoly Observatory Budapest 16 October 1990 HU ISSN 0374 - 0676 PHOTOELECTRIC MINIMA TIMES OF GK CEPHEI AND VARIATIONS IN ITS PERIOD The eclipsing binary star GK Cep (BV 382, BD+70d 1183, HD 205372, SAO 10069) has been observed photoelectrically during 1987 and 1988 from the National Observatory of Athens, Greece. The observations were made with a two-beam, multi-mode, nebular-stellar photometer attached to the 48-inches Cassegrain reflector at the Kryonerion Astronomical Station. Reduction of the observations has been made as usual (Hardie, 1962) and the pass bands of the B and V filters used are in close accordance to the standard ones. The stars HD 205328 and HD 205284 were used for comparison and checking, respectively. From our observations of GK Cep eleven new minima times (8 primaries and 3 secondaries) were derived using Kwee and Van Woerden's (1956) method and are the mean values of B and V colours. They are given in Table I, the successive columns of which give: the Hel. JD, the differences (O-C)_x and (O-C)_xx, the minimum type and the number N of the observed points. In the residuals (O-C)_x and (O-C)_xx the C's have been calculated using the following ephemeris formulae, respectively: (I): Min I=2438694.7063+0.936157d.E (due to Dworak, 1975) and (II): Min I=2438694.683+0.9361669d.E (Krakow No. 57, 1986) TABLE I (New photoelectric minima times of GK Cephei) Hel.JD (O-C)_x (O-C)_xx Min N 2440000.+ days days Type 7063.5472 0.0654 0.0002 II 13 7064.4828 0.0678 -0.0004 II 18 7065.4193 0.0652 0.0000 II 17 7335.5029 0.0675 -0.0006 I 18 7449.5456 0.0678 -0.0004 I 18 7350.4812 0.0673 -0.0010 I 21 7351.4189 0.0688 0.0006 I 21 7394.4846 0.0713 0.0026 I 26 7395.4176 0.0671 -0.0006 I 31 7396.3564 0.0708 0.0023 I 31 7397.2882 0.0664 -0.0023 I 22 As is obvious from the (O-C) values presented in Table I, the period of GK Cep has changed and we tried to find out how this happened. Thus, from all the minima times of GK: Cep found in the literature, Table II was made, the successive columns of which give: the Hel. JD; the residuals (O-C)_x, in which the C's have been calculated using ephemeris formula given by the foregoing equation (I); the number E of the cycles passed; the minimum type; the kind of observation and the reference. TABLE II (Minima Times of GK Cephei, found in the literature) Hel.JD (O-C)= E Min Obs. References 2430000.+ days Type kind 8606.7068 +0.0007 -94 I pe Ruiz et al. 1967 8622.636 +0.014 -77 I pe " 8627.7712 -0.0054 -71.5 II pe Gleim, 1967 8634.795 +0.003 -64 I pe Ruiz et al. 1967 8642.759 +0.004 -55.5 II pe " 8644.635 +0.008 -53.5 II pe " 8649.757 -0.014 -48 I pe " 8652.585 +0.006 -45 I pe " 8652.588 +0.009 -45 I pe " 8694.7047 -0.0016 0 I pe " 8694.7070 +0.0007 0 I pe Gleim, 1967 8700.7885 -0.0083 6.5 II pe Ruiz et al. 1967 8703.6015 -0.0038 9.5 II pe " 8708.7487 -0.0001 15 I pe " 8709.6866 +0.0018 16 I pe Gleim, 1967 8711.5587 +0.0016 18 I pe Ruiz et al. 1987 8731.6864 -0.0034 39.5 II pe Gleim, 1967 8748.5433 +0.0025 57.5 II pe " 8997.0911 +0.0061 323 I pe Ruiz et al. 1967 8997.5592 +0.0006 323.5 II pe " 9405.7367 +0.0137 759.5 II pe " 2440000.+ 0532.3881 +0.0056 1963 I pe Dworak, 1975 0532.3883 +0.0058 1963 I pe " 0542.2229 +0.0053 1973.5 II pe " 0542.2237 +0.0061 1973.5 II pe " 0845.5325 0.0000 2297.5 II pe " 0845.5327 +0.0002 2297.5 II pe " 0852.5476 -0.0006 2305 I pe " 0852.5488 +0.0006 2305 I pe " 0867.5257 -0.0010 2321 I pe " 0867.5265 -0.0002 2321 I pe " 0868.4625 -0.0003 2322 I pe " 0868.4632 +0.0004 2322 I pe " 0869.3972 -0.0018 2323 I pe " 0869.3976 -0.0014 2323 I pe " 0870.3347 -0.0005 2324 I pe Gleim, 1967 0870.3353 +0.0001 2324 I pe " 0876.4243 -0.0014 2330.5 II pe " 0876.4247 -0.0010 2330.5 II pe " 1080.5062 -0.0017 2548.5 II pe " 1080.5082 +0.0003 2548.5 II pe " 1969.9191 +0.0034 3474 I pe Isles, 1985 2685.5421 +0.0025 4231 I pe Dworak, 1976 3068.446 +0.0142 4672 I v BBSAG No. 31, 1977 3069.388 +0.0200 4673 I v " 3090.446 +0.0145 4695.5 II v " 3098.400 +0.0112 4704 I v " 3099.338 +0.0130 4705 I v " 3100.279 +0.0189 4706 I v " 3106.366 +0.0188 4712.5 II v BBSAG No. 32, 1977 3107.302 +0.0198 4713.5 II v BBSAG No. 31, 1977 3159.246 +0.0070 4769 I v BBSAG No. 32, 1977 3173.426 +0.0016 4784 I v " 3369.398** -0.0163** 4993.5 II v BBSAG No. 37, 1978 3392.362* +0.0199 5018 * II* v " 3399.375 +0.0117 5025.5 II v " 3464.439 +0.0128 5095 I v BBSAG No. 35, 1977 3488.314** -0.0204** 5121.5 II v BBSAG No. 36, 1978 3517.327 +0.0079 5151.5 II v " 3589.399* -0.0042 5228.5* I* v BBSAG No. 38, 1978 3590.360* +0.0207 5229.5* I* v " 4937.516 +0.047 6668.5 II pe Isles, 1985 5671.4680 +0.0517 7452.5 II pe " 5890.536 +0.0589 7686.5 II v BBSAG No. 73, 1984 6162.4943 +0.0636 7977 I pe Isles, 1985 6236.383 -0.0041 8056 I v BBSAG No. 77, 1985 6279.450 -0.0003 8102 I v " 6287.4712 +0.0635 8110.5 II pe Pohl et al. 1987 6311.3413 +0.0616 8136 I pe " 6318.3636 +0.0636 8143.5 II pe " 6325.3847 +0.0627 8151 I pe " 6332.4057 +0.0625 8158.S II pe " 6339.4287 +0.0643 8166 I pe " 6691.4264 +0.0670 8542 I pe " 7695.453 +0.065 9614.5 II pe BBSAG No. 93, 1990 7804.444** -0.0061** 9731 I v " 7805.387** +0.0008** 9732 I v " Notations: * They found to be of different type than that reported in the reference ** They haven't taken into account in the diagrams 1,2 and 3 since their (O-C) values deviate too much from the others. From the data presented in Table II, three visual minima types must be of different kind than that referred in the literature, as the number E of the cycles passed indicates. These, have been marked with an asterisk. Moreover, there are four visual minima times, marked with two asterisks, in which the (O-C)_x values deviate too much from all the others and they haven't been taken into account. From the data of Tables I and II, the diagrams, 1,2 and 3 were drawn, in which linear-linear, linear-quadratic and quadratic least square fitting has been achieved, respectively, and where crosses denote the photoelectric minima times, while squares the visual ones. Unfortunately, there are not observed minima times for GK Cep from JD 2443000 till 2445000, and from the visual ones which cover only the interval from JD 2443100 till 2443900, two have not taken into account, since they deviate. As one can see from the diagrams 1,2 and 3 the period of GK Cep from JD 2458000 till 2443000 was almost constant, but after that it seems that there is a jump. More observations of its minima time are needed to see if this period increasing will be continued. [FIGURE 1] Figure 1 The (O-C) diagram of GK Cep according to Dworak's (1975) ephemeris formula. Linear-linear least squares fitting. [FIGURE 2] Figure 2 The (O-C) diagram of GK Cep according to Dworak's (1975) ephemeris formula. Linear-quadratic least squares fitting. [FIGURE 3] Figure 3 The (O-C) diagram of GK Cep according to Dworak's (1975) ephemeris formula. Quadratic least squares fitting. H. ROVITHIS-LIVANIOU and P. ROVITHIS and P.G. NIARCHOS A. KALIMERIS Section of Astrophysics Astronomical Institute Astronomy and Mechanics National Observatory of Athens University Athens, P.O. Box 20048 GR 157 83 Zografos Athens 118 10, Greece. Athens, Greece. References: Dworak, T.Z.: 1975, Acta Astron, 25, 103. [BIBCODE 1975AcA....25..103D ] Dworak, T.Z.: 1976, I.B.V.S. No. 1081. Gleim, J.K.: 1967 Astron. J. 72, 510. [BIBCODE 1967AJ.....72..503G ] Hardie, R.H.: 1962, in W.A. Hiltner (ed.), Stars and Stellar Systems Vol. II, Astronomical Techniques, The University of Chicago Press, Chicago. Isles, J.E.: 1985, I.B.V.S. No. 2741. Kwee, K.K. and Van Woerden, H.: 1956, Bull. Astron. Inst. Neth. 12, 327. [BIBCODE 1956BAN....12..327K ] Rocznik Astronomiczny Observatorium Krakowskiego, No. 57, p.100, Krakow, 1986. Ruiz, J.J. and Stokes, A.: 1967, I.B.V.S. No. 180.