Quasi-normal modes of spherically symmetric black hole spacetimes with   cosmic string in a Dirac field
  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. 
Sini R, Nijo Varghese, V. C. Kuriakose