  Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, as these objects cannot be explained otherwise
without introducing new physics. However, there is no observational evidence
that the space-time around them is really described by the Kerr solution. The
Kerr black hole hypothesis can be tested with the already available X-ray data
by extending the continuum-fitting method, a technique currently used by
astronomers to estimate the spins of stellar-mass black hole candidates. In
general, we cannot put a constraint on possible deviations from the Kerr
geometry, but only on some combination between these deviations and the spin.
The measurement of the radio power of transient jets in black hole binaries can
potentially break this degeneracy, thus allowing for testing the Kerr-nature of
these objects.
