  An invariant definition of mass in asymptotically de-Sitter space-times is
given that relies on the existence of a time-like Killing vector on a sphere
surrounding the mass but does not require going to an asymptotic region. In
particular the mass can be calculated exactly on a sphere inside the
cosmological horizon. The formalism requires varying the background metric
solution by a perturbation that satisfies the linearized equations of motion
but need not share the Killing symmetry of the solution and is therefore
ideally suited to calculating masses in stationary space-times perturbed by a
gravitational wave without going beyond the cosmological horizon.
