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The visual binary star ADS 15571 has been observed with the 26" refractor at Pulkovo since 1960. Periodic perturbations in the relative motion of the components are studied in this paper. The visible motion of the component B was resolved into orbital motion of the barycenter relative to component A and an elliptical motion of the photocenter relative to the barycenter. The visible ellipse of the photocenter motion is used to determine the elements of the orbit by the direct geometric method of A. A. Kiselev and estimate the perturbing mass. The minimum mass of the invisible companion is 0.62 ¤ M .

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Photographic observations of the visual binary star ADS 7251 were made from 1962 through 2006 on the 26-inch refractor at the Pulkovo Observatory. 206 plates were obtained and used to derive about 3000 relative positions. A uniform series of positions of the B-A components of ADS 7251 with errors in one annual average distance ρ and position angle θ of ±0".006 and ±0 o .02, respectively, have been used to determine the elements of the orbit and the sum of the masses of the components. The orbit has been determined using a method of apparent motion parameters developed at Pulkovo. The positions and motions of the components A and B relative to a system of reference stars have also been calculated from 146 of the plates; this made it possible to determine the position of the center of mass and the ratio of the masses of the components. The masses of A and B are 0.57 ¤ M and 0.53 ¤ M , respectively, a result that is consistent with their spectral classes and does not indicate the presence of significant excess mass.

Abstract-We have used two astrometric methods developed at the Main Astronomical Observatory of the Russian Academy of Sciences-the method of apparent-motion parameters (AMP) and a direct geometrical method (DGM)-to derive the orbit of the star S2 around the Galactic center, and thereby the mass of the supermassive black hole at the Galactic center. The AMP method, which is based on measurements of the curvature of a fairly short orbital arc, is efficient if observational data on the relative radial velocity are available. The mass of the supermassive black hole was also estimated using astrophysical methods, based on the empirical relation between the masses of the supermassive black holes at the centers of galaxies and quasars and the radio and X-ray luminosities of these regions. We estimate the magnetic-field strength near the event horizon of the supermassive black hole at the Galactic center using a synchrotron self-absorption model. PACS numbers : 98.35.Jk, 98.38.Am