  Cosmic strings that are attached to rapidly spinning black holes can extract
significant amounts of rotational energy and angular momentum. Here we study
the effect on primordial black holes, which are expected to form with one or
more cosmic strings attached. Although large primordial black holes are
predicted to rapidly spin up due to accretion soon after forming, we argue that
cosmic strings will spin them down again. As a result, the spins of primordial
black holes should consequently be observed to be near zero. We also
investigate the effect on a supermassive black hole of capturing a cosmic
string and the possibility of observing the subsequent spin down by its effect
on a pulsar orbiting the black hole.
