  Symmetries compatible with asymptotic flatness and admitting gravitational
and electromagnetic radiation are studied by using the Bondi-Sachs-van der Burg
formalism. It is shown that in axially symmetric electrovacuum spacetimes in
which at least locally a smooth null infinity in the sense of Penrose exists,
the only second allowable symmetry is either the translational symmetry or the
boost symmetry. Translationally invariant spacetimes with in general a straight
"cosmic string" along the axis of symmetry are non-radiative although they can
have a non-vanishing news function. The boost-rotation symmetric spacetimes are
radiative. They describe "uniformly accelerated charged particles" or black
holes which in general may also be rotating - the axial and an additional
Killing vector are not assumed to be hypersurface orthogonal. The general
functional forms of both gravitational and electromagnetic news functions, and
of the mass aspect and total mass of asymptotically flat boost-rotation
symmetric spacetimes at null infinity are obtained. The expressions for the
mass are new even in the case of vacuum boost-rotation symmetric spacetimes
with hypersurface orthogonal Killing vectors. In Appendices some errors
appearing in previous works are corrected.
