  We consider the Hassan-Rosen bimetric field equations in vacuum when the two
metrics share a single common null direction in a spherically symmetric
configuration. By solving these equations, we obtain a class of exact solutions
of the generalized Vaidya type parametrized by an arbitrary function. Besides
not being asymptotically flat, the found solutions are nonstationary admitting
only three global spacelike Killing vector fields which are the generators of
spatial rotations. Hence, these are spherically symmetric bimetric vacuum
solutions with the minimal number of isometries. The absence of staticity
formally disproves an analogue statement to Birkhoff's theorem in the
ghost-free bimetric theory which would state that a spherically symmetric
solution is necessarily static in empty space.
