COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3867 Konkoly Observatory Budapest 14 April 1993 HU ISSN 0374 - 0676 BF DRACONIS The star BD+69d 1006 = BV379 = BF Dra was reported as an eclipsing variable by Strohmeier, Knigge and Ott (1962). Doppner (1962) established the elements of variation from photographic photometry and gave a finder chart. These elements were further refined by Strohmeier, Knigge and Ott (1963), yielding JDhel, min = 2436317.579 + 5.60545 * E. (1) With these data, BF Dra was included in the GCVS (Kholopov et al., 1985). As far as we know, no complete photoelectric light curve of this rather bright variable has been published. The only three photoelectric measurements known to us were obtained by Lacy (1992) in order to establish the mean brightness at maximum light and the mean colours of BF Dra (V=9.823+-0.005, B-V = 0.489+-0.005, U-B = -0.022 +-0.003). Between 1988 and 1993, we have secured a set of photoelectric observations covering the whole light curve of BF Dra in blue light, as well as complete photoelectric coverage of the primary minimum in the visual part of the spectrum from three different observatory sites. RD and MW used the 35cm Schmidt-Cassegrain telescope of R. Scafraniec Observatory in Metzerlen, Switzerland in connection with a commercial, uncooled, photon-counting photometer of the "Starlight-1" type. This photometer is equipped with an EMI 9924A photomultiplier tube and a filter set very close to the standard Johnson UBV system. FA observed with his private, automated telescope at Zweikirchen, Germany, also a 35cm Schmidt-Cassegrain system outfitted with an uncooled photometer tube (EMI 9781A) and a set of filters matching the standard BV closely. Finally, MW collected more data at the Skalnate Pleso Observatory (1780m above sea level), Slovak Republik, employing a 60 cm reflecting telescope with a single-channel pulse-counting photoelectric photometer. We have deposited the whole list of observations (262 in B, 127 in V) in the IAU Commission 27 Archives of Unpublished Photoelectric Photometry (Breger, 1988). All the data are in the instrumental system, with no correction for differential extinction applied. Since the comparison star is located close to the variable, this simplification is well founded. The general good agreement of the three different and independent sets of observations to within +-0.02 mag in both colours supports this conclusion. In Figure 1 we show the whole set of blue observations, using the elements determined from the photoelectrically observed minima JDhel, Min = 2447276.3948 + 11.211079 * E. (2) For reasons that will be discussed below, the period given by Strohmeier et al. (1960) had to be approximately doubled. [FIGURE 1] Figure 1: Photoelectric blue light curve of BF Draconis in the instrumental system. Differential magnitudes in the sense variable minus comparison star (HD176086 = SAO009270 = BD+70d1034) are shown. BF Dra exhibits the typical light curve of a well detached binary system. The two minima are narrow and, because a difference in minimum brightness of 0.12+-0.02 mag is clearly discernible, well established. The BV photometry of FA during primary minimum shows, that the value of B-V changes only by 0.02 mag. Therefore, we can infer, that the two components in the system have about the same spectral type. The mean colours given by Lacy (1992) are indicative of a pair of unreddened stars of spectral type F7V, while the GCVS classifies BF Dra as an F8 star. In Figure 2, we give the primary minimum, both in B and V, while Figure 3 shows the secondary minimum in blue light. It is immediately evident, that the two minima are of unequal duration, and that the secondary is displaced from phase 0.5 by a small amount. Although the secondary minimum is not sufficiently covered by observation, we find the following parameters describing the two minima of BF Dra: Psi_min = 0.484 +- 0.002; A_B,Min I = 0.57 +- 0.01; A_B,Min II = 0.45 +- 0.02 D_I=0.028p+-0.001=0.314d+-0.011 D_II=0.056p+-0.005=0.63d+-0.06 These results indicate a slight ellipticity of the orbit of BF Dra. From the data given above, we have tried to derive the parameters of this orbit by applying the arguments of Martynov (1973): [FIGURE 2] Figure 2: Primary minimum of BF Draconis in blue (above) and in visual light. [FIGURE 3] Figure 3: Secondary minimum of BF Draconis in blue light Considering the spectral type (F7V), the length of the period, the narrow eclipses and the nearly equal depth of both minima, we can assume the orbital inclination i to be very close to 90d. From the phase of secondary minimum and the ratio D_II : D_I, we derive the values for the excentricity e = 0.33 and for the current longitude of the periastron omega = 94d. Further observations are needed to clarify the exact physical nature of this interesting star. R. Diethelm M. Wolf F. Agerer Astr. Institut Astron. Institute Dorfstrasse 19 Universitat Basel Charles University D-W-8311 Zweikirchen Venusstrasse 7 Svedska 8 Germany CH-4102 Binningen 150 00 Praha 5 Switzerland Czech Republic References: Breger, M: 1988, PASP 100, 751 [BIBCODE 1988PASP..100..751B ] Doppner, M: 1962, MVS No. 699 Kholopov P.N. et al.: 1985, General Cat. of Variable Stars, 4th ed., Moskow Lacy, C. H: 1992, AJ 104, 801 [BIBCODE 1992AJ....104..801L ] Martynov D. Y: 1973, in ,Eclipsing Variable Stars", editor: B. V. Tsesevich, John Wiley and Sons, New York - Toronto, p. 270 Strohmeier W., Knigge, R., Ott, H: 1962, Bamb. Veroffentl. V, No. 14 Strohmeier W., Knigge, R., Ott, H: 1963, Bamb. Veroffentl. 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