2007PASP..119..349N

Several stars orbit around a black hole candidate of mass 3.7x10⁶ M_☉ in the region of the Galactic center. Looking for general relativistic (GR) periastron shifts is limited by the existence of a stellar cluster around the black hole, which modifies the cluster orbits due to classical effects that might mask the GR effect. Only if one knows the cluster parameters (its mass and core radius) it is possible to unequivocally deduce the expected GR effects and then test them. In this paper, we show that the observation of the proper motion of Sgr A* (v_SgrA*^_=0.4±0.9 km/s; Reid & Bruthaler) could help us to constrain the cluster parameters significantly, and that future measurements of the periastron shifts for at least three stars may adequately determine the cluster parameters and the mass of the black hole.