External stability for Spherically Symmetric Solutions in Lorentz   Breaking Massive Gravity
  We discuss spherically symmetric solutions for point-like sources in Lorentz-breaking massive gravity theories. This analysis is valid for   St\"uckelberg's effective field theory formulation, for Lorentz Breaking Massive Bigravity and general extensions of gravity leading to an extra term $-Sr^{\gamma}$ added to the Newtonian potential. The approach consists in analyzing the stability of the geodesic equations, at the first order (deviation equation). The main result is a strong constrain in the space of parameters of the theories. This motivates higher order analysis of geodesic perturbations in order to understand if a class of spherically symmetric Lorentz-breaking massive gravity solutions, for self-gravitating systems, exists. Stable and phenomenologically acceptable solutions are discussed in the no-trivial case $S\neq 0$. 
Andrea Addazi and Salvatore Capozziello
gr-qc astro-ph.HE hep-th