Behaviour of Singularities of the Kerr-Newman and the Kerr-Sen Solutions   by Arbitrary Boost
  Behaviour of singularities of the rotating black holes under an arbitrary boost is considered on the basis of a complex representation of the Kerr theorem. We give a simple algorithm allowing to get explicit expressions for the metric and the position of the singularities for arbitrary direction and magnitude of the boost, including the ultrarelativistic case. The non-smoothness of the ultrarelativistic limit is discussed. The Kerr-Sen BH-solution to low energy string theory is also analyzed. 
Alexander Burinskii (NSI Russian Academy of Sciences) and Giulio Magli   (Dipartimento di Matematica del Politecnico di Milano)