  We show that, though they are rare, there are asymptotically flat space-times
that possess null geodesic congruences that are both asymptotically shear- free
and twist-free (surface forming). In particular, we display the class of
space-times that possess this property and demonstrate how these congruences
can be found. A special case within this class are the Robinson- Trautman
space-times. In addition, we show that in each case the congruence is isolated
in the sense that there are no other neighboring congruences with this dual
property.
