COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3102 Konkoly Observatory Budapest 21 October 1987 HU ISSN 0374 - 0676 BV LIGHT CURVES OF BY Dra IN 1986 BY Dra (= BD +51 2402 = HD 234677 = SAO 31048) in the prototype binary system (M0Ve + M0Ve) of the flare stars which also shows low-amplitude quasi-periodic light variability outside of flares. This variability is generally attributed to rotational modulation of starspots (Rodono` 1986). Previous photometry of this very interesting star was presented by Rodono` et al. (1983). We present BV differential photometry carried out during the period 8 July - 8 November 1986, on 22 nights with a single-channel photon-counting photometer fed by a 0.91m Cassegrain telescope. Our principal comparison star was HD 172268 (= BD +51 2408 = SAO 31070), while HD 172468 (=BD +51 2410 = SAO 31077) was observed several times each night, as check star. [FIGURE 1] Figure 1 V, B-V light curves of BY Dra obtained at Catania Observatory during the period July - November 1986. Phases were computed from the ephemeris given by Chugainov (1966): JDhel = 2438983.612 + 3.836xE. Table I Photometric observations of BY Dra JD(hel) PHASE V B-V 2446000.0+ 619.4549 .5743 8.18 1.19 620.4535 .8346 8.22 1.19 621.4944 .1060 8.23 1.21 622.4653 .3590 8.19 1.20 623.4556 .6172 8.19 1.20 624.4722 .8822 8.23 1.21 678.4204 .9459 8.23 1.20 682.5171 .0138 8.23 1.20 683.4565 .2586 8.20 1.21 685.3637 .7559 8.22 1.20 686.4045 .0272 8.23 1.21 687.5217 .3186 8.19 1.20 688.5217 .5793 8.18 1.21 708.4042 .7623 8.20 1.20 709.3779 .0161 8.24 1.19 711.4557 .5578 8.18 1.20 714.3657 .3164 8.20 1.21 716.3697 .8388 8.22 1.21 730.3324 .4787 8.18 1.21 731.2870 .7236 8.21 1.20 742.2400 .5829 8.18 1.20 743.2319 .8415 8.22 1.10 The observations were corrected for atmospheric extinction, Nightly mean BV differential magnitudes (variable - comparison star) were computed with reference to several standard stars. The following values of the V magnitude and B-V color for the comparison star BD +51 2408 were assumed: V=7.893 H-V=1.259 (Vogt, 1975). Mean JDhel, V magnitudes and B-V color of BY Dra are listed in Table I. Photometric phases, also listed in Table I, were computed using the photometric ephemeris derived from Chugainov (1966): JD(hel) = 2438983.612 + 3.836xE The resulting V light curve and B-V color are presented in Fig. 1. No significant B-V color variation is observed. Nightly standard deviations for V and B-V data are 0.01 and 0.015 mag, respectively. We also obtained spot models for the BY Dra V light curve, assuming two spotted circular areas and using our computer code based on the analytical method outlined by Friedemann and Gurtler (1975) and already presented by Rodono` et al.(1986). We adopted the following values for the parameter involved in the model: i (inclination of the star rotation axis with respect to the line of sight) = 30 degree; mu (limb darkening coefficient) = 0.85; Ls/Lp (luminosity ratio between the secondary and the primary component of the system) = 0.52; V0 (unspotted V magnitude) = 8.00; Ts (photospheric temperature) = 4100 Tspot (spot temperature) = 3500 K; The resulting latitudes for the two spots are 81 and 25 degrees. The radii are 38 and 13 degrees respectively. The two spots are 295 degrees apart in longitude (Figure 2). [FIGURE 2] Figure 2 Results of two-spot model fit for the observed light variations (upper panel). The bottom panel shows the contribution of each spot, arrows indicate the phase of light maximum (v) and light minimum (^); the corresponding spot configurations are presented on the right hand panel. In previous papers (Vogt 1981, Melkonian et al. 1981, Olah and Soliman 1984), the question concerning the variability of the comparison stars HD 172468 and HD 172268 has been raised. From our observations of BY Dra in the years 1984, 1985 and 1986 the following seasonal average Delta mag, in the sense HD 172268 minus HD 172468, result: 1984 DeltaV= .378 +/- .005 DeltaB = .370 +/- .006 1985 DeltaV= .375 .006 DeltaB = .372 .006 1986 DeltaV= .372 .007 DeltaB = .369 .007 Although no systematic variation can be inferred from these observations, we sometimes obtained nightly mean values that seem to indicate some variability. At present time we are not able to ascertain of one, or both, of the comparison are variable. Nevertheless, as obtained by Olah and Soliman (1984), the light curves generally show higher scatter using HD 172268 as comparison, but further dedicated observations of these two stars are necessary. We wish to thank Prof. Marcello Rodono for suggesting to carry out the present observations and for stimulating discussion. C. CUTISPOTO, G. LETO, I. PAGANO, S. SANTAGATI, R. VENTURA Catania Astrophysical Observatory Astronomical Institute of Catania University CNR-Gruppo Nazionale di Astronomia, Unita di Ricerca di Catania V. le A. Doria, 6 95125 Catania, ITALY References: Chugainov, P.F.: 1966, Inf.Bull.Var.Stars, No. 122. Friedemann, C., and Gurtler, J.: 1975, Astron.Nachr. 296, p. 125. [BIBCODE 1975AN....296..125F ] Melkonian, A.S., Olah, K., Oskanian, A.V., Oskanian, V.S.: 1981, Astrophys. 17, p.112. [BIBCODE 1981Ap.....17..112M ] Olah, K., and Soliman, M.A.: 1984, Inf. Bull.Var.Stars, No. 2648 Rodono, M. Pazzani, V. and Cutispoto, G.: 1983, in "Activity in Red-Dwarf Stars", IAU Coll. No. 71., p. 179, P. Byrne and M. Rodono eds., D.Reidel, Dordrecht, Holland. [BIBCODE 1983ards.proc..179R ] Rodono, M.: 1986, in "Highlights of Astronomy", p. 429. J.P. Swings ed., D.Reidel, Dordrecht, Holland [BIBCODE 1986HiA.....7..429R ] Rodono, M. et al.: 1986, Astron.Astrophys. 165, p. 135 [BIBCODE 1986A&A...165..135R ] Vogt, S.S.: 1975, Astrophys.J. 199, p. 418. [BIBCODE 1975ApJ...199..418V ] Vogt, S.S.: 1981, Astrophys.J. 250, p. 327. [BIBCODE 1981ApJ...250..327V ]