Collision of spinning black holes in the close limit: The parallel spin   case
  In this paper we consider the collision of black holes with parallel spins using first order perturbation theory of rotating black holes (Teukolsky formalism). The black holes are assumed to be close to each other, initially non boosted and spinning slowly. We estimate the properties of the gravitational radiation released from such an collision. The same problem was studied recently by Gleiser {\em et al.} in the context of the Zerilli perturbation formalism and our results for waveforms, energy and angular momentum radiated agree very well with the results presented in that work. 
Gaurav Khanna