  The gravitational waves emitted (some time) after two black holes merge are
well described by the theory of linear perturbations on a spacetime
characterized by the mass and spin of the remnant. However, in the very early
stages right after merger, both the mass and spin are changing. In this work we
explore, in a set up based on Vaidya's spacetime, the dynamical consequences of
a change of mass in the spacetime due to the accretion of null matter (for
example, gravitational waves). We show that accretion imprints time-dependent
frequencies and amplitude to a ringdown waveform, and we show how to model
accurately this effect in certain regimes. We also comment on the direct
emission of gravitational waves due to perturbations in the in--falling matter,
which is of relevance for black holes embedded in astrophysical environments.
