  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.
