  In 4D general relativity, the angular momentum of a black hole is limited by
the Kerr bound. We suggest that in string theory, this bound can be breached
and compact black-hole-like objects can spin faster. Near such "superspinars,"
the efficiency of energy transfer from the accreting matter to radiation can
reach 100%, compared to the maximum efficiency of 42% of the extremal Kerr (or
6% of the Schwarzschild) black hole. Finding such superspinning objects as
active galactic nuclei, GBHCs, or sources of gamma ray bursts, could be viewed
as experimental support for string theory.
