On space-times admitting shear-free, irrotational, geodesic null   congruences
  Space-times admitting a shear-free, irrotational, geodesic null congruence are studied. Attention is focused on those space-times in which the gravitational field is a combination of a perfect fluid and null radiation. 
A.M. Sintes, A.A. Coley, D.J. McManus