Axionic Black Holes and an Aharonov-Bohm Effect for Strings

It is shown that a black hole of fixed mass can carry arbitrary axionic charge. The unique static black-hole solution is found to have vanishing axion field strength but nonvanishing potential. The axion charge cannot be detected by point particles, but can be detected by strings in a process analogous to the Aharonov-Bohm effect. It is argued that the existence of axion charge may play a significant role in the late stages of black-hole evaporation.