COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2838 Konkoly Observatory Budapest 20 December 1985 HU ISSN 0374-0676 PHOTOELECTRIC OBSERVATION OF DK CYGNI The variable DK Cygni was discovered by Guthnick and Prager (1927). Their photographic light curve indicates that the system is of W Ursae Majoris type. Visual light curves were published later and photoelectric observation were made by Hinderer (1960) and by Binnendijk (1964). Rucinski and Kaluzny (1981) obtained five photoelectric observations of DK Cyg during a survey of W UMa- type systems. In the present study the variable was observed photoelectrically in U,B,V colours with the 50 cm Cassegrain telescope of Konkoly Observatory. A total of 213 observations in yellow light, 211 observations in blue light and 211 observations in ultraviolet light were made on five nights during 1985. The composite light curves in different colours are displayed in Fig.1. The time of minimum for both the primary and secondary minimum was determined by the method of Hertzsprung (1928). A time of the primary minimum, which is rather uncertain, was determined from Rucinski and Kaluzny's observations using our mean light curve. The times of minima are listed in Table I. The O-C's were computed from Binnendijk's ephemeris: J.D.Hel.Min.I = C1 = 2437999.5838 + 0.47069055d E Our parabolic fit of the O-C diagram (Fig.2) shows that the orbital period of the system is increasing. Binnendijk (1964) has already pointed out the variability of the period, but did not mention any detail about its nature. [FIGURE 1] Figure 1. The composite light curves of DK Cygni in different colours [FIGURE 2] Figure 2. O-C diagram of DK Cygni Table I J.D.Min. Type E (O-C)1 (O-C)2 Type weight Rem. Ref. of min. of obs. 24760.039 I -28129 +.039 0.000 pg 2 G,P 1 26568.423 I -24286 +.030 +.002 v 1 Ts 2 28068.5032 I -21099 +.0193 -.001 v 1 Pi 3 33539.316 I - 9476 -.004 -.007 v 1 Sz 4 33889.512 I - 8732 -.003 -.005 v 1 Sz 4 34179.462 I - 8116 +.003 +.001 v 1 Sz 5 34458.584 I - 7523 +.005 +.004 v 1 Sz 6 34606.380 I - 7209 +.004 +.003 v 1 Sz 6 34668.503 I - 7077 -.004 -.005 v 1 Kl 7 34702.402 I - 7005 +.006 -.005 v 1 Sa 6 34988.578 I - 6397 +.002 +.001 v 1 Sz 6 35303.470 I - 5728 +.002 +.002 v 1 Sz 8 35762.391 I - 4753 -.001 0.000 pe 3 Hi 9 36163.421 I - 3901 +.001 +.002 v 1 Sz 10 36453.353 I - 3285 -.012 -.012 v 1 Sz 11 37995.5831 II - 8.5 +.0002 +.001 pe 3 Bi 12 37999.5838 I 0 0.000 +.001 pe 3 Bi 12 38000.5257 I + 2 +.0005 +.002 pe 3 Bi 12 43081.6367: I +10797 +.007: +.001: pe 3 pp 13 46300.4635 II +17635.5 +.0165 -.001 pe 3 pp - 46303.5245 I +17642 +.0180 0.000 pe 3 pp - G,P = Guthnick and Prager 1 (1927); Ts=Tsesevich 2 (1950); Pi=Piotrowski 3 (1936); Sz=Szafraniec 4 (1952), 5 (1953), 6 (1955) 8 (1956), 10 (1958), 11 (1959); Kl=Klepikova, see Tsesevich 7 (1954); Hi=Hinderer 9 (1960); Bi=Binnendijk (1964) ; Rucinski and Kaluzny 13 (1981). The second-order weighted least-squares solution of the O - C values calculated by eq. (1) yielded the following ephemeris: J.D.Hel.Min.I=C2=2437999.5828 + 0.47069066 E + 5.39 x 10^-11 E^2 (2) +-.0007 +- .00000005 +- .26 The steady increase of the period is: 1.078 x 10^-10 days/cycle +-.052 The secondary minimum observed on 2446300 is 0.05m brighter than the primary one. Comparing our observations with those of Binnendijk (1964) one can see that the maximum light before the secondary minimum is brighter by about 0.05m in our light curve. This part of our light curve was observed on 2446250. A systematic difference at this phase of the light curve was already noted by Binnendijk (1964). M. PAPARO Konkoly Observatory HUNGARY M. A. HAMDY Helwan Observatory EGYPT I. JANKOVICS Konkoly Observatory HUNGARY References: Binnendijk, L., 1964, Astron. J. 69, 157. [BIBCODE 1964AJ.....69..157B ] Guthnick, P., Prager, R., 1927, Astron. Nachr. 229, 455. 1927, Kleinere Veroff. Berlin-Babelsberg No. 4,9. Hertzsprung, E., 1928, Bull. Astron. Inst. Neth. 4, 179. [BIBCODE 1928BAN.....4..172H ] Hinderer, F., 1960, J. des Observateurs 43, 161. [BIBCODE 1960JO.....43..161H ] Piotrowski, S., 1936, Bull. Obs. Astron. Belgrade 1,9. Rucinski, S.M., Kaluzny, J., 1981, Acta Astronomica 31, 409. [BIBCODE 1981AcA....31..409R ] Szafraniec, R., 1952, Acta Astronomica Ser. c. 5, 8. 1953, ibid. 5, 52. 1955, ibid. 5, 190. 1956, ibid. 6, 142. [BIBCODE 1956AcA.....6..141S ] 1958, ibid. 8, 190. [BIBCODE 1958AcA.....8..189S ] 1959, ibid. 9, 48. [BIBCODE 1959AcA.....9...48S ] Tsesevich, V.P., 1950, Astron. Circ. USSR No. 100, 18 1954, Odessa Izv. 4, 258.