  The S-Stars in the Galactic-center region are found to be on near-perfect
Keplerian orbits around presumably a supermassive black hole, with periods of
15-50 yr. Since these stars reach a few percent of light speed at pericenter,
various relativistic effects are expected, and have been discussed in the
literature. We argue that an elegant test of the Einstein equivalence principle
should be possible with existing instruments, through spectroscopic monitoring
of an S-star concentrated during the months around pericenter, supplemented
with an already-adequate astrometric determination of the inclination. In
essence, the spectrum of an S-star can be considered a heterogeneous ensemble
of clocks in a freely-falling frame, which near pericenter is moving at
relativistic speeds.
