Gravitational wave solutions of quadratic curvature gravity using a null   coframe formulation
  Quadratic curvature gravity equations are projected to a complex null coframe by using the algebra of exterior forms and expressed in terms of the spinor quantities defined originally by Newman and Penrose. As an application, a new family of impulsive gravitational wave solutions propagating in various type D backgrounds is introduced. 
Ahmet Baykal
gr-qc