Covariant constraints for generic massive gravity and analysis of its   characteristics
  We perform a covariant constraint analysis of massive gravity valid for its entire parameter space, demonstrating that the model generically propagates five degrees of freedom; this is also verified by a new and streamlined Hamiltonian description. The constraint's covariant expression permits computation of the model's caustics. Although new features such as the dynamical Riemann tensor appear in the characteristic matrix, the model still exhibits the pathologies uncovered in earlier work: superluminality and likely acausalities. 
S. Deser, M. Sandora, A. Waldron and G. Zahariade
hep-th gr-qc