COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3055 Konkoly Observatory Budapest 29 July 1987 HU ISSN 0374-0676 UBV PHOTOMETRIC OBSERVATIONS OF DY PEGASI DY Pegasi is a well known dwarf cepheid. The variability of this star was discovered by Morgenroth (1934) and it was often observed to study the nature and the period of its light variations. The observations obtained and the investigations published between 1934 and 1980 were summarized by Mahdy and Szeidl (1980), Soloviev (1938, 1940) who determined the first elements of the light variation of this star, indicated that the star had strong light curve variation. This variation was especially considerable in heights of light maxima (0.3 magn.). Although Lange (1944) questioned it, later on Grigorevsky and Mandell (1960) found significant light curve variation and suggested a period of 0.2554 day for this secondary variation, (This modulation period yields a period of 0.0567 day for the first harmonic mode and a period ratio P1/P0 = 0.778.) Modern photometric observations (Masani and Broglia, 1954; Hardie and Geilker, 1958; Broglia, 1961; Geyer and Hoffmann, 1975) have shown some non-repetitive character from cycle to cycle of the light curves but the changes in the heights of maxima have never exceeded 0.05 magn. Karetnikov and Medvedev's (1964) photometric observations, however, showed that the form of the light curve of DY Peg changed significantly and very rapidly. According to their investigation the variations in the amplitude exceeded 0.4 magn. Karetnikov and Medvedev proposed a period of 0.255413 day for the secondary variation. The period changes of DY Peg have been discussed by Quigley and Africano (1979) and by Mahdy and Szeidl (1980) in detail. In the latter study about 100 times of maxima of DY Peg observed by photographic or photoelectric method have been collected and analyzed. The O - C residuals could be almost equally well approximated either by two straight lines or by a quadratic formula. In the first case the period has been constant with a sudden decrease of 7.5 x 10^-8 day = -6.5 ms around J.D. 2437500 while in the other case the period has continuously decreased by -7.6 x 10^-13day.cycle^-1 = 0.33 ms.year^-1. Since both problems, the multiple periodicity and period changes of DY Peg which are essential in investigating the physics of dwarf cepheids are unsolved we decided to observe the star again in order to carry out a new period analysis and to investigate the possible light curve variation. The 453 photoelectric observations were made at Kottamia Observatory, Egypt on the nights 2/3, 3/4, 4/5 and 6/7 August 1986 (J.D. 2446645, 2446646, 2446647 and 2446649). The one beam photoelectric photometer attached to the 74 inch telescope had an EMI 9558B photomultiplier. The amplified output of the tube was fed into a strip chart recorder. The U, B and V filters used were very close to the standard system of Johnson and Morgan. A number of standard stars were also observed to determine the extinction coefficients and the transformation constants. The star BD + 16d 4878 served as the comparison and BD + 16d 4876 as the check star. The light curves observed in V and B light are shown in Figures 1 - 4. As can be seen from these figures the light curves are repeated fairly regularly but they also show some non-repetitive character from cycle to cycle. In the last three columns of Table I we have also indicated the brightness of the observed maxima in all the three colours as compared to the comparison. star. Although the deviations are certainly larger than the observational errors, the variation at maximum in every colour is less than 0.04 magn. In a forthcoming paper we will carry out a detailed period analysis searching for secondary oscillation in this star. From our observational material we could derive eight new times of light maximum of DY Peg and we extended the baseline of coverage to 48 years. These times of maxima are listed in Table I. We have not found any significant shift in time between the yellow, blue and ultraviolet maxima therefore each time of light maximum given in Table I is a mean value obtained from the yellow, blue and ultraviolet light curve. [FIGURE 1] [FIGURE 2] [FIGURE 3] [FIGURE 4] Table I Observed times and heights of maxima of DY Peg Hel. max. J.D. E (O-C)l (O-C)s DeltaVmax DeltaBmax DeltaUmax 2446000+ 645.4466 190514 -0.0004 +0.0001 +0.186 -0.157 -0.061 645.5195 190515 -0.0004 +0.0001 +0.180 -0.155 -0.060 646.4675 190528 -0.0005 0.0000 +0.147 -0.153 -0.042 646.5408 190529 -0.0001 +0.0004 +0.147 -0.161 -0.034 647.4155 190541 -0.0005 0.0000 +0.181 -0.151 -0.037 649.3849 190568 -0.0001 +0.0004 +0.167 -0.161 -0.042 649.4578 190569 -0.0001 +0.0003 +0.148 -0.153 -0.038 649.5307 190570 -0.0002 +0.0003 +0.185 -0.145 -0.035 The list of light maxima given by Mahdy and' Szeidl (1980) has been supplemented by the times of maxima of Table I and both linear and second order least - squares solutions have been carried out. Mahdy and Szeidl (1980) suggested that a sudden decrease in the period of DY Peg might take place around J.D.2437500 therefore we used only those light maxima to the linear fit which have been observed since that time. The least- squares solution gave the linear ephemeris: Cl (Max. hel.) = J.D. 2432751.9655 + 0.072926302d*E If the period had really a sudden change around J.D. 2437500, and before and after it the period was constant, the value of the period change was DeltaP = -7.0 x 10^-8 day. The second order fit using all the maxima observed photographically or photoelectrically yielded the following formula: Cs (Max. hel.) = J.D. 2432751.9614 + 0.072926365dE - 2.31d x 10^-13E^2. In this case beta = -4.62 x 10^-13 day*cycle 1 = -6.34 x 10^-12days*day^-1 = -20.0 ms*century^-1. The corresponding O-C residuals are given in Table I under the headings (O-C)l and (O-C)s. At present we cannot decide which of the two approximations is correct. Further observations can only settle this important question. The observation and investigation of DY Peg will be continued in the frame of the current observational program of dwarf cepheids at Kottamia Observatory. HAMED A. MAHDY Helwan Observatory Helwan, Egypt References: Broglia, P. 1961, Mem. Soc. astr. It. 32. 7 [BIBCODE 1961MmSAI..32....7B ] Geyer, E.H. and Hoffman, M., 1975, Astr. Astrophys. Suppl. 21. 183 [BIBCODE 1975A&AS...21..183G ] Grigorevsky, V.M. and Mandell, O.E., 1960, Per. Zvezdy 13. 190 [BIBCODE 1960PZ.....13..190G ] Hardie, R.H. and Geilker, C.D., 1958, Astrophys. J. 127. 606 [BIBCODE 1958ApJ...127..606H ] Lange, G.A., 1944, Astr. Tsirk, No. 28.3 Mahdy, H.A. and Szeidl, B., 1980, Mitt. Sternw. Ung. Akad. Wiss., Budapest, No. 74 [BIBCODE 1980CoKon..74....1M ] [CoKon No. 74] Masani, A, and Broglia, P., 1954, Mem. Soc. astr. It. 25. 431 [BIBCODE 1954MmSAI..25..431M ] Morgenroth, O., 1934, Astron. Nachr. 252. 389 [BIBCODE 1934AN....252..389M ] Quigley, R. and Africano, J., 1979, Publ. Astr. Soc. Pacific 91. 230 [BIBCODE 1979PASP...91..230Q ] Soloviev, A. V., 1938, Tadjik Obs, Tsirk. No. 37 Soloviev, A.V., 1940, Per. Zvezdy 5. 340