  We derive closed formulas for the mass and spin parameters of a Kerr black
hole in terms of a minimal quantity of observational data: the red-/blue-shifts
of photons emitted by massive particles (stars) moving on geodesics around the
black hole and their respective orbital radius. It turns out that given a set
of two (three) stars revolving around the black hole, these formulas involve
just eight (twelve) observational data. For the case of a single star orbiting
the black hole we need a minimal set of four observational measurements to
analytically determine both parameters.
