  Quasi-periodic oscillations (QPOs) are a common feature in the X-ray flux of
stellar-mass black hole candidates, but their exact origin is not yet known.
Recently, some authors have pointed out that data of GRO J1655-40
simultaneously show three QPOs that nicely fit in the relativistic precession
model. However, they find an estimate of the spin parameter that disagrees with
the measurement of the disk's thermal spectrum. In the present work, I explore
the possibility of using the relativistic precession model to test the nature
of the black hole candidate in GRO J1655-40. If properly understood, QPOs may
become a quite powerful tool to probe the spacetime geometry around black hole
candidates, especially if used in combination with other techniques. It turns
out that the measurements of the relativistic precession model and of the
disk's thermal spectrum may be consistent if we admit that the black hole
candidate in GRO J1655-40 is not of the Kerr type.
