  Spontaneous violation of Lorentz symmetry by the vacuum condensation of an
antisymmetric $2$-tensor is considered. The coset construction for nonlinear
realization of spacetime symmetries is employed to build the most general
low-energy effective action for the Goldstone modes interacting with photons.
We analyze the model within the context of the Standard-Model Extension and
noncommutative QED. Experimental bounds for some parameters of the model are
discussed, and we readdress the subtle issues of stability and causality in
Lorentz non-invariant scenarios. Besides the two photon polarizations, just one
Goldstone mode must be dynamical to set a sensible low-energy effective model,
and the enhancement of the stability by accounting interaction terms points to
a protection against observational Lorentz violation.
