  Dirac equation for a general black hole metric having a cosmic string is
derived. The quasi-normal mode frequencies for Schwarzschild, RN extremal, SdS
and near extremal SdS black hole space-times with cosmic string perturbed by a
massless Dirac field are obtained using WKB approximation and found that in all
these cases, decay is less in black holes having cosmic string compared to
black holes with out string.
