Null infinity and Killing fields

A recent investigation due to Ashtekar and Xanthopoulos of isometrics compatible with asymptotic flatness at null infinity is extended by considering space-times which admit "only a piece of ℐ". This extension makes the interplay between asymptotic conditions and permissible isometrics more transparent by bringing out how the various parts of the asymptotic structure individually lead to restrictions on isometrics and by separating the consequences of field equations from those of purely geometric asymptotic conditions.