COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 1460 Konkoly Observatory Budapest 1978 August 18 ON THE PERIOD CHANGE OF CEPHEID BD +56d2806 The history of investigation of the star BD +56d2806 is short. Fernie and Hube (ApJ 168, 437, 1971) discovered that it is located on the H-R diagram in the instability strip or near it. Percy (IBVS No. 983, 1975) made 6 BV-observations and ranked BD +56d2806 in cepheids with period about 3 days. Using 28 BV- observations Szabados (IBVS No. 1107, 1976) confirmed that this star belonged to the cepheids and derived P = 2d.80591. Our photographic light estimates (a total about 1200) from Dushanbe, Odessa and Moscow plates led to the detection of a large period change (Figure 1). The period variation during 1940-1975 may be represented by instantaneous elements: JD Max JD 2400000+ 2400000+ P 29160-34300 30615.57 2.80511d (1) 35310-40885 35749.12 2.80556 (2) 42020-42790 42676.397 2.80618 (3) or by progressive increase at the mean rate (DeltaP/P)_100=0.001. E and O-C of the normal moments of light maxima in Table 1 and Figure 1 are calculated from the elements (2). Let us attract attention to the single point at E=-6922 on the O-C diagram. 13 light estimates in the interval JD 2414663-18235 which form this point do not allow miscalculation of epochs. This conclusion follows from the fact that these estimates are not brought together to a sufficient mean light curve with either P=2d.80463 or P=2d.80494 which should occur in the case of epoch miscalculation either at parabolic or linear O-C variations, respectively. P=2d.80556 gives a sufficient mean light curve. The form of the light curve (Figure 2) and a large period change characterize BD +56d2806 as the star W Vir. Then taking their luminosity from Kukarkin-Rastorguev's P-L relation and using Parengo's method of the interstellar absorption correction with Sharov's parameters revised we derive the intrinsic colour (B-V)^o=0m.64. V_med and (B-V)_med were taken from Szabados (1976). Table 1 Times of normal light maxima Max JD E O-C 2400000+ (periods) 16329.86 13pg M -6922 +0.295 27686.98: 5pg M -2874 +0.370 28508.67 14pg M -2581 +0.250 29675.91 18pg M -2165 +0.295 30615.57 16pg D -1830 +0.223 30624.06 9pg M -1827 +0.249 30983.07 28pg D -1699 +0.213 31302.86 12pg D -1585 +0.197 31675.95 15pg D -1452 +0.179 32029.32 2pg D -1326 +0.133 32789.55 19pg D -1055 +0.105 33193.09 14pg D(Kodak) - 929 +0.119 33148.59 20pg D - 927 +0.080 33378.73 32pg D(Kodak) - 845 +0.110 33524.44 98pg D - 793 +0.046 33555.37 27pg D(Isochrom)- 782 +0.071 33863.90 46pg D - 672 +0.041 34253.80 18pg D -.533 +0.015 35749.12 26pg D 0 0.000 36088.68 58pg D,Od + 121 +0.031 36461.84 52pg D,Od + 254 +0,038 36475.76:: 32pv Od + 259 0.000 36823.59 37pg D,Od + 383 -0.021 37193.95 31pg D,Od + 515 -0.005 37208.18:: 12pv Od + 520 +0.060 37586.80 35pg D,Od + 655 +0.014 37901.08:: 30pv Od + 767 +0.034 37914.93 45pg D,Od + 772 -0.029 38277.05:: 13pv Od + 901 +0.043 38299.36 44pg D,Od + 909 -0.005 38664.10 48pg D,Od +1039 +0.001 38936.15:: 25pv Od +1136 -0.031 38981.07 55pg D,Od +1152 -0.020 39371.07 14pg D +1291 -0.010 39721.77 22pg D +1416 -0.008 40103.33 15pg D +1552 -0.007 40111.70:: 22pv Od +1555 -0.023 40473.68 23pg D +1684 -0.001 40824.38 23pg D +1809 +0.001 41054.48:: 36pv Od +1891 +0.016 41949.60:: 39pv Od +2210 +0.069 42030.95 6 B +2239 +0.065 Percy(1975) .98 6 V " +0.075 " 42639.93:: 18pv Od +2456 +0.126 42676.397 28 V +2469 +0.125 Szabados .425 28 B " +0.135 (1976) Remarks: used plates D - Dushanbe, Od - Odessa, M - Moscow. [FIGURE 1] O-C diagram of BD +56d2806. It was constructed from the elements (2). Dots show photographic maxima, open circles - photovisual ones, filled squares - photoelectric B ones, open squares - photoelectric V ones. [FIGURE 2] The mean light curve of BD +56d2806. Dots show the normal points derived from 528 observations in JD 2435310-40885 with the elements (2), open circles - ones from 318 observations in JD 2430251-34299 with elements (1). We conclude that BD +56d2806 is very similar to the W Vir star - AU Peg (Erleksova, in press in Variable Stars) with the following characteristics: P value, smallness of light amplitude, smallness of light curve asymmetry, large (B-V)^o, unusual P behaviour. The latter is in the fact that the constancy of P during early epochs is interrupted sudden decrease, then P begins its rapid progressive increase. Both stars are of interest from the point of view of stellar evolution, since both stars are on the red edge of the instability strip and show rapid progressive increase in their periods. I should like to thank Prof. V.P. Tsessevich and Dr. A.S. Sharov for providing oppurtunity to use Odessa plates and light estimates from Moscow plates. G.E. ERLEKSOVA Dushanbe Astrophysical Institute of Tadjik Academy of Sciences, USSR