COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3780 Konkoly Observatory Budapest 30 September 1992 HU ISSN 0374 - 0676 UBV PHOTOMETRY OF THE SYMBIOTIC BINARY BF CYGNI The light-curve (LC) of the symbiotic star BF Cygni has been studied since 1890 photographically (e.g. Jacchia, 1941). It is characterized by several outbursts of 2-3 mag amplitude accompanied by a gradual decrease of the star's brightness, from m_pg approx 10 to m_pg approx 11, and by long period variations, Delta m_pg approx 1, corresponding to an eclipse-like effect. Recent observations were collected by Hric et al. (1991) and Skopal et al. (1992). The last increase of the brightness became at the middle of 1987 (m_v approx 10.6). After a gradual decrease up to m_v approx 12.2 at the beginning of 1989, the star's brightness grew abruptly to m_v approx 9.8 at the end of 1989. Our photometric observations of BF Cyg have been made during this recent outburst phase, from November 1989. The measuring were carried out in the standard UBV system using a one-channel photoelectric photometer installed in the Cassegrain focus of the 0.6/7.5 m reflector of the Skalnate Pleso and Stara Lesna (near Tatranska Lomnica) Observatories, operating on the principle of the method of pulse counting. The stars HD 183650 (SAO 68384), V = 6.96, B-V = 0.71, Sp G5 and BD+30d3594, V = 9.54, B-V = 1.20, U-B = 1.70 were used as comparisons. Several observations were obtained within the framework of an international campaign of long-term observations of symbiotic stars (Hric et al. 1991; Skopal et al. 1992). Figure 1 shows the UBV photometry of BF Cyg. During the whole observational period BF Cyg was brightest in the U filter. This fact reflects a strong interaction in the binary. In October 1990 the star's brightness reached the maximum 9.35 in the U filter, but during the period June-August, 1991 the brightness faded by about 1.8 mag in all filters (U approx 11.2, B approx 12.0, V approx 11.5) and in September 1991 sudden brightening by about 0.9 mag was observed. Such behaviour of the LG reflects an eclipse-like effect. Compiling our V measurements with the visual magnitude estimations, the middle of this primary minimum can be derived at JD 2448444 +/- 1.1 days by the second degree polynomial least squares fit. Behaviour of the UBV light curves around JD 2448130 shows a possible existence of a secondary eclipse (hot component between the cool component and observer). Their middle at JD 2448129.4 +/- 3.4 was determined by the same method as for the primary minimum. According to Heintz (1978), the position of the secondary, t_s, and the primary, t_p, minima. gives the condition which most be satisfied by the orbital eccentricity e, periastron angle omega and orbital period P of binary as pi(t_s - t_p - P/2)/P = 2 e cos omega (1) Orbital period P = 757 days (e.g. Pucinskas 1970), t_s = 24486.4 and t_p = JD 24484444 imply e cos omega = 0.133 (2) [FIGURE 1] Figure 1: Light curves for BF Cygni according to the data published by Hric et al. (1991) and Skopal et al. (1992) [FIGURE 2] Figure 2: Relationship between the O-C values and the star's brightness (see text in detail). The minima were taken from Jacchia (1941) - full circles, and determined from data published by Kudashkina (1988) - open circles This value agrees with the orbit geometry derived by Mikolajewska et al. (1989) from radial velocities of the emission HeI lines (e = 0.23 +/- 0.10, omega = 60d +/- 27d). Position of our last primary minimum indicates new average value of the orbital period of 759 days. Jacchia (1941) and Pucinskas (1970) showed considerable real differences, up to 100 days, between observed and computed minima positions. From this point of view it is very problematical to determine a more accurate orbital period (if such one exists at all) of this binary system. Generally, this irregularity results from interaction in the system. For example, if we take the maximum of the star's brightness before and after the minimum observed as a parameter characterising interaction in the system, we can obtain the relationship between this parameter and the O-C value (Fig. 2). The times of the primary minima were taken from Jacchia (1941) and the O-C values were determined according to the ephemeris JD_Min = 2415035.7 + 759.3 E (3) derived from the first well defined minimum, at JD 2415795 in Jacchia (1941) and average value of the orbital period derived from this and recent minimum. This fact means that during the maxima of the activity the period is larger than during the quiescence. Analogical problem of the orbital period determination is known, for example, in the symbiotic star AG Peg, and/or the symbiotic star V 1329 Cyg exhibited eclipses with 950 day separation (Grygar et al. 1979) during the quiescent phase (m_pg approx 15), but during the outburst phase (m^max_pg approx 12 to 14) the orbital period became longer, about 964 days (Nussbaumer et al. 1986). Detailed analysis of BF Cyg LC can lead us to solving this crucial problem. Augustin SKOPAL Astronomical Institute Slovak Academy of Sciences 059 60 Tatranska Lomnica Czechoslovakia References: Grygar J., Hric L., Chochol D., Mammano A. 1979, Bull. Astron. Inst. Czechosl. 30, 308 [BIBCODE 1979BAICz..30..308G ] Heintz W.D. 1978, Double Stars, D. Reidel Publ. Co., Dordrecht, Holland, 115 [BIBCODE 1978GAM....15.....H ] Hric L., Skopal A., Urban Z., Dapergolas A., Hanzl D., Isles J.E., Niarchos P., Papousek J., Pigulski A., Velic Z. 1991, Contrib. Astron. Obs. Skalnate Pleso, 21, 303 [BIBCODE 1991CoSka..21..303H ] Jacchia, L. 1941, Bull. Harv. Coll. Obs. No. 915 Kudashkina, L.S. 1988, Peremennye Zvezdy, 22, 679 [BIBCODE 1988PZ.....22..679K ] Mikolajewska M., Kenyon S.J., Mikolajewski M. 1989, Astron. J. 98, 1427 [BIBCODE 1989AJ.....98.1427M ] Nussbaumer H., Schmutz W., Vogel M. 1986 A&A, 169, 154 [BIBCODE 1986A&A...169..154N ] Pucinskas A. 1970, Bull. Vilnius Univ. Astron. Obs. No. 27, 24 [BIBCODE 1970VilOB..27...24P ] Skopal A., Hric L., Urban Z., Pigulski A., Blanco C., Papousek J., Hanzl D., Agerer F., Niarchos P., Rovithis-Livaniou H., Rovithis P., Tsvetkova K., Semkov E., Velic Z., Michalek F., Komacka L., Schweitzer E., Korth S., 1992, Contrib. Astron. Obs. Skalnate Pleso, 22, 131 [BIBCODE 1992CoSka..22..131S ]