COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3777 Konkoly Observatory Budapest 29 September 1992 HU ISSN 0374 - 0676 ORBITAL LIGHT CURVE AND PARAMETERS OF X-RAY NOVA GS2023+338=V404 Cyg The X-ray nova G52023+338=V404 Cyg has been discovered as the best black hole candidate by recent spectroscopic investigations (Charles, 1991; Casares et al., 1992) with the orbital period P = 6.47259d +/- 0.0009d and mass function M^3_x sin^3 i F_v(M) = --------------- = 6.26 M_sun. (M_x + M_v)^2 In this paper we present the optical orbital light curve of V404 Cyg in quiet stage (V = 19m) that is in good agreement with the radial velocity curve (Casares et al., 1992) in the model of a close binary system. Photometric observations have been obtained at the 50-cm telescope equipped with a television device in Crimean Astrophysical Observatory (Abramenko et al., 1983). During 5 nights 170 individual observations were obtained in 1991 when the system V404 Cygni was at quiet stage (V=19m). The convolution of the average nightly values of the light of V404 Cyg with the spectroscopic orbital period (Casares et al., 1992) P=6.47259d and zero phase JD 2448477.353 (optical G-K star is in front) is presented in Figure 1. The mean light curve is given in Table 1. The magnitude differences refer to the brighter comparison star (No. 1 in Figure 2). A considerable (approx. 0.3m) ellipticity effect of the optical G-K star is observed. Minima in the light of V404 Cyg are observed at the phases 0 and 0.5, which proves the fact that the radial velocity curve obtained for the G-K star by Casares et al. (1992) reflects the orbital motion of this star. So our photometric data proves correctness of the mass function determination for V404 Cyg f_v(M) = -6.26 M_sun obtained by Charles (1991) and Casares et al. (1992). Because of the considerable ellipticity effect observed, we can conclude that the G-K star is the donor contributing the accreting matter onto the black hole. Interpretation of the optical light curve of V404 Cyg in the framework of the standard model of X-ray binaries (Antokhina, 1988; Goncharsky et al., 1991) allows us to estimate the values of the parameters of V404 Cyg binary system: inclination of the orbital plane i = 45deg-70deg; mass ratio q = M_x/M_v = 4.5 - 2.6 accordingly; orbital separation of the components: a = 46-39 R_sun, mass of the black hole candidate M_x = 27 - 14 M_sun, mass of the visible star (which according to our results is a K0III star with the radius of about 12 R_sun, but not G9V star) is M_v = 6 - 5 M_sun. The computer simulated model of V404 Cyg is presented in Figure 3. Our results confirm the conclusion of Charles (1991) and Casares et al. (1992) that V404 Cyg is the best black hole candidate. [FIGURE 1] Figure 1. Observed (points) and theoretical (line) light curve of V404 Cyg. The theoretical curve corresponds to the mass ratio q=3 and inclination i=70deg. [FIGURE 2] Figure 2. Finding chart for V404 Cyg. The magnitude of the star No. 1 is 19.65B, that of the star No. 2 is 20.4B. Table I. Mean light curve of V404 Cyg. Phase Delta m sigma N 0.00 0.04m 0.02m 43 0.17 -0.11 0.09 11 0.31 -0.44 0.02 33 0.46 0.01 0.02 31 0.85 -0.12 0.03 9 0.86 -0.15 0.02 23 [FIGURE 3] Figure 3. Computer simulated picture of the model for V404 Cyg E. A. ANTOKHINA, A. M. CHEREPASHCHUK, S. Yu. SHUGAROV Sternberg State Astronomical Institute Moscow, Russia E. P. PAVLENKO Crimean Astrophysical Observatory Ukraine References: Abramenko, A. N., Prokofyeva, V. V., Bondar, N. I., et al.: 1983, Izv. Crimean Astrophys. Obs., 78, 182 Antokhina, E. A.: 1988, Astron. Zh., 65, 1164 [BIBCODE 1988AZh....65.1164A ] Casares, J., Charles, P. A., Naylor, T.: 1992, Nature, 355, 6361 [BIBCODE 1992Natur.355..614C ] Charles, P. A.: 1991, Gemini, No. 34, 11 [BIBCODE 1991Gemin..34...10C ] Goncharsky, A. V., Romanov, S. Yu., Cherepashchuk, A. M.: 1991, Limit-Number Parametric Inverse Problems of Astrophysics, Moscow, Univ. Press [BIBCODE 1991fipa.book.....G ]