  We consider an effective model formed by usual QED (minimal coupling) with
the addition of a nonminimal Lorentz violating interaction (proportional to a
fixed 4-vector $b_\mu$) which may radiatively generate both CPT even and odd
terms in the pure gauge sector.
  We show that gauge invariance from usual QED, considered as a limit of the
model for $b_\mu \rightarrow 0$, plays an important role in the discussion of
the radiatively induced Lorentz violating terms at one-loop order. Moreover,
despite the nonrenormalizability of the (effective) model preventing us from
readily extending our discussion to higher orders, it is still possible to
display the general form of the breaking terms of the photon sector in the on
shell limit organized in powers of $b_\mu$ which in turn can be considered as a
small expansion parameter.
