  Yang's theorem states that an initial J=1 state cannot decay into two
photons. Because of this result some reactions relating to elementary particles
or atomic transitions can be ruled out. The theorem is not valid in the
presence of background electric or magnetic fields. In this work we show that
the decay of a J=1 particle into two photons is permitted by Bose symmetry and
rotational invariance when the background of the decay process is not pure
vacuum but contains an external classical magnetic/electric field. We also
discuss constraints on these amplitudes from {\bf CP} invariance.
