  We derive consistent equations for gravitational wave oscillations in
bigravity. In this framework a second dynamical tensor field is introduced in
addition to General Relativity and coupled such that one massless and one
massive linear combination arise. Only one of the two tensors is the physical
metric coupling to matter, and thus the basis in which gravitational waves
propagate is different from the basis where the wave is produced and detected.
Therefore, one should expect -- in analogy to neutrino oscillations -- to
observe an oscillatory behavior. We show how this behavior arises explicitly,
discuss phenomenological implications and present new limits on the graviton
parameter space in bigravity.
