COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2728 Konkoly Observatory Budapest 23 May 1985 HU ISSN 0374 - 0676 HD 115781: A LARGE-AMPLITUDE ELLIPSOIDAL VARIABLE We began photometry of HD 115781 ( = BD +34deg2411) after it appeared on a list of bright suspected variables (Hall 1983). The HD spectral type is G5 and the magnitudes (Haggkvist and Oja 1973) are V=5.13mag, B-V=1.14mag, U-B = 0.94mag. According to Griffin and Fekel (1985) it is a spectroscopic binary with an orbital ephemeris of JD(hel.) = 2445280.30 + 18.6917d n, (1) +/- .03 +/- .0012 where the initial epoch is a time of maximum positive radial velocity. As shown in Table I, differential photometry was obtained on a total of 40 nights in 1984 at three different observatories. The comparison star was HD 115707 = BD +33deg2324, which is only 40 arc minutes away. Nightly means, of the three individual measures obtained on each night, are given in Table II. The first 31 are Lines, the next 5 Barksdale, the last 4 Stelzer. Each value of DeltaV, in the sense variable minus comparison, has been corrected for differential atmospheric extinction and transformed differentially to V of the UBV system. It was immediately obvious that the brightness was varying nearly sinusoidally with a period of 9 or 10 days. By linear least squares we fit 14 times of maximum and minimum brightness, obtained from the light curve graphically, with the ephemeris JD(hel.) = 2445803.8 + 9.31d n, (2) +/- .3 +/- .06 where the initial epoch is a time of maximum brightness. Twice our value of the period is 18.62d+/-0.12d, consistent with the value in equation (1) found by Griffin and Fekel. This made us suspect that HD 115781 is varying as a result of the ellipticity effect. The suspicion is confirmed by the following reasoning. Times of maximum positive radial velocity should (in a circular orbit) correspond to times of quadrature, which should be times when an ellipsoidal variable reaches maximum brightness. The initial epoch in equation (1), brought forward by exactly 28 cycles, is JD(hel.) 2445803.668+/-0.045d. This differs from the epoch in equation (2) by only 0.1d, entirely consistent with the relevant uncertainties. Table I Tally of Observations Observer Location Telescope Nights lambda Barksdale Florida 14-inch 5 V Lines Arizona 20-inch 31 VB Stelzer Illinois 14-inch 4 V [FIGURE 1] Figure 1 Light curve of HD 115781 in V, where Delta is in the sense variable minus HD 115707 and phase is computed with the ephemeris in equation (3) in which zero phase is at maximum brightness. The 0.16m variation probably results from the ellipticity effect and there is no evidence of eclipses, the reflection effect, or an RS CVn-type asymmetry. Figure 1 is a plot of the DeltaV values in Table II, where phase is computed with the ephemeris JD(hel.) = 2445803.8 + 18.69d n. (3) The light curve shape is characteristic of the ellipticity effect, although we note the 0.16mag amplitude is relatively large. Unequal depths of the two minima would indicate the differential reflection effect, but there is no clear indication of this. Unequal heights of the two maxima might Table II Nightly Mean Differential V Magnitudes of HD 115781 JD(hel.) JD(hel.) JD(hel.) 2445000+ DeltaV 2445000+ DeltaV 2445000+ DeltaV 804.7847 -0.310mag 844.7055 -0.171mag 874.6761 -0.208mag 805.7056 - .270 846.7116 - .191 875.6865 - .241 806.7222 - .220 847.6935 - .242 889.6958 - .289 807.7332 - .185 848.6983 - .298 813.6990 - .345 854.7096 - .205 797.6914 - .219 814.6885 - .301 859.7009 - .318 839.6753 - .294 820.6834 - .274 861.7098 - .281 840.6733 - .313 826.7171 - .178 862.7076 - .228 853.6440 - .232 827.6865 - .177 864.7179 - .170 854.6914 - .177 828.7211 - .223 869.6772 - .328 830.6884 - .338 870.6757 - .284 830.6704 - .304 832.7035 - .337 871.6833 - .252 832.6528 - .310 840.7068 - .322 872.6839 - .222 836.6543 - .156 841.7245 -0.315 873.6762 -0.200 841.6504 -0.323 indicate the "wave" seen in RS CVn-type variables, but there is only a slight suggestion of this and, moreover, we could not find in the literature any report of the Ca II H and K emission which would indicate that HD 115781 is an RS CVn system. Eclipses would show up as anomalous faint points around 0.25P and/or 0.75P, but we see no evidence of this. The DeltaB measures made by Lines are not reported here, but analysis did show that there is no significant change in B-V index as this variable goes through its 18.69d cycle. We thank Griffin and Fekel very much for sharing results of their spectroscopic investigation in advance of publication. And we thank the National Science Foundation for support provided through research grant AST 84-14594. RICHARD D. LINES 6030 North 17th Place Phoenix, Arizona 85016 WILLIAM S. BARKSDALE 633 Balmoral Road Winter Park, Florida 32789 HAROLD J. STELZER 1223 Ashland Avenue River Forest, Illinois 60305 DOUGLAS S. HALL Dyer Observatory Vanderbilt University Nashville, Tennessee 37235 References: Griffin, R. and Fekel, F. C. 1985, in preparation. Haggkvist, L. and Oja, T. 1973, Astr. Astrophys. Suppl. 12, 381. [BIBCODE 1973A&AS...12..381H ] Hall, D. S. 1983, I.A.P.P.P. Comm. No. 13, 6. [BIBCODE 1983IAPPP..13....6H ]