COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2891 Konkoly Observatory Budapest 7 May 1986 HU ISSN 0374-0676 PHOTOMETRIC VARIABILITY OF 27 CYGNI 27 Cygni (HR 7689, HD 191026, K0 IV, V = 5.36m) is used as a "red standard" in a long-term UBV photometric campaign on Be stars (Harmanec, Horn and Koubsky 1982). The star also appears on a list of forty suspected variable stars (Fekel and Hall 1985). Several groups of astronomers were therefore observing the star in 1985, and in this paper, we present our combined data. These observations were made with several telescopes. Basic information about the observers, instruments and observational technique used is summarized in Table I. All the observations were reduced to differential magnitudes on the standard UBV system. The analysis of the observations was further complicated by the fact that not all observers used the same comparison stars. Some used 22 Cyg (HR 7613, HD 188892), the local comparison star in the Be star campaign; others used 21 Cyg (HR 7615, HD 188947), which was suggested by Fekel and Hall. The constancy of 22 Cyg has been checked by repeated use of the check star 36 Cyg, but the constancy of 21 Cyg has not yet been checked. Neither of these two is a suitable comparison for 27 Cyg in fact. 22 Cyg is a blue star and its long-term constancy has yet to be verified. 21 Cyg is a suspected variable (CSV 101914 = NSV 12586) and a multiple system of at least 5 stars. Considering the high incidence of variables among the brighter stars in Cygnus, any future observations should be carried out with at least two different check stars until the present situation is clarified. 15 Cyg and 39 or 42 Cyg appear at the moment as the best available candidates for a comparison and a check star, respectively. The values of (V, B-V) given by Nicolet (1978) for 22 Cyg and 21 Cyg are (4.94, -0.08) and (3.89, +1.02). Harmanec, Horn and Koubsky, in the Bright Be Star Observing Programme, Release 5, prefer (4.95, -0.09) for Table I Observers Observatory Photometer, Filters Comparison Check Telescope Tube Harmanec, Horn Hvar DC UBV 22Cyg 36Cyg Koubsky, Kriz, 0.65-m EMI Bozic Cassegrain Hoff and Hillside Starlight 1 Kelsey 0.4-m PC BV 21Cyg - reflector Landis Landis DC V 21Cyg - 0.2-m IP21 Newtonian Percy and Univ. DC BV 22Cyg 36Cyg Richer of Toronto 0.4-m Cassegrain Reisenweber Rolling Ridge Optec V 22Cyg - 0.2-m SSP-3 21Cyg Celestron solid state compound Wasson Sunset Hills PC V 21Cyg - 0.2-m reflector 22 Cyg, and we have adopted those values. Because of potential problems in combining observations from many and varied sources, we adopted the following procedures: the (V,B-V) observations obtained at Hvar Observatory were adopted as standard, and all other observations were fitted to them. This required a small adjustment of +0.010m to the (B-V) values of Percy and Richer, an adjustment of -0.03m to the V values of Hoff and Kelsey, and an adjustment of +0.042m to the V values of Reisenweber using 22 Cyg as the comparison. The light curve obtained using the values so obtained is shown in Figure 1, together with the corresponding data for the check star. The variability amounts to about 0.05m in V. A similar range of variation can also be found in the previous published all-sky photometry of 27 Cyg (c.f. Blanco et al. 1970). The variability is apparent in all of the long sets of data, though it is less apparent in the combined data [FIGURE 1] Figure 1: The light and colour curves of 27 Cyg (upper and middle panel) and the light curve of the comparison star 22 Cyg (lower panel). Slightly smaller symbols represent points of slightly lower accuracy. because of the extra scatter contained therein. The behaviour does not appear to be strictly periodic, although no period analysis has been attempted at this stage. Variations seem to have a characteristic time scale of about 50-60 days. The cause may be pulsation, or more likely the rotation of a spotted star. However, more rapid variations may also be suspected in some data sets, and should be checked by future observations. A part of these rapid fluctuations is probably connected with the relatively low signal-to-noise ratio of the data (as evidenced by the plot for the check star), but the variability of the comparison star 21 Cyg also cannot be excluded at present. Radial velocity observations of the star are sparse, but there are no reports of velocity variations. Further photometric and spectroscopic observations of this bright variable star are certainly warranted. The observations have been deposited in File 187 in the Archive of Unpublished Photoelectric Observations, maintained by IAU Commission 27 (Breger 1982). Acknowledgement: This work was supported in part by the Natural Sciences and Engineering Research Council of Canada. *J.R. PERCY and M.G. RICHER Department of Astronomy University of Toronto Toronto, Ontario Canada M5S 1A1 H. BOZIC Hvar Observatory Faculty of Geodesy University of Zagreb 41000 Zagreb Yugoslavia P. HARMANEC, J. HORN, P. KOUBSKY and S. KRIZ Astronomical Institute Czechoslovak Academy of Sciences 251 65 Ondrejov Czechoslovakia L.A. KELSEY and D.B. HOFF Hillside Observatory University of Northern Iowa Cedar Falls IA 50613 USA H.J. LANDIS Landis Observatory 50 Price Road West Locust Grove GA 30248 USA R.C. REISENWEBER Rolling Ridge Observatory 3621 Ridge Parkway Erie PA 16510 USA R. WASSON Sunset Hills Observatory 15870 Del Prado Drive Hacienda Heights CA 91745 USA *to whom any correspondence about this paper should be addressed References: Blanco, V.M., Demers, S., Douglas, G.G. and FitzGerald, M.P. 1970 Publ. U.S. Naval Obs. 21 (2nd series) [BIBCODE 1968PUSNO..21....1B ] Breger, M. 1982 IAU Inf. Bull. Var. Stars # 2246 Fekel, F.C., Hall, D.S. 1985 IAPPP Comm. #20, 37 [BIBCODE 1985IAPPP..20...37F ] Harmanec, P., Horn, J., Koubsky, P. 1982 IAU Symp. 98: Be Stars, ed. M. Jaschek and H.-G. Groth (Dordrecht: D. Reidel), 269 [BIBCODE 1982IAUS...98..269H ] Nicolet, B. 1978 Catalogue of Homogeneous Data in UBV Photoelectric Photometry (Strasbourg: Strasbourg Observatory) [BIBCODE 1978ppch.book.....N ] [DATAFILE 2] [From IBVS 2908] E R R A T U M Figure 1 of the No. 2891 issue of the IBVS was in error. The correct version of this figure is given below. [FIGURE 1b] Figure 1: The light and colour curves of 27 Cyg (upper and middle panel) and the light curve of the comparison star 36 Cyg (lower panel). The constancy of the latter shows that the primary comparison star 22 Cyg is also constant. Slightly smaller symbols represent points of slightly lower accuracy.