A rotating incompressible perfect fluid space-time
  A rigidly rotating incompressible perfect fluid solution of Einstein's gravitational equations is discussed. The Petrov type is D, and the metric admits a four-parameter isometry group. The Gaussian curvature of the constant-pressure surfaces is positive and they have two ring-shaped cusps. 
Zolt\'an Perj\'es, Gyula Fodor, L\'aszl\'o \'A. Gergely and Mattias   Marklund