  The ability to model the evolution of compact binaries from the inspiral to
coalescence is central to gravitational wave astronomy. Current waveform
catalogues are built from vacuum binary black hole models, by evolving Einstein
equations numerically and complementing them with knowledge from slow-motion
expansions. Much less is known about the coalescence process in the presence of
matter, or in theories other than General Relativity. Here, we explore the
Close Limit Approximation as a powerful tool to understand the coalescence
process in general setups. In particular, we study the head-on collision of two
equal-mass, compact but horizonless objects. Our results show the appearance of
"echoes" and indicate that a significant fraction of the merger energy goes
into these late-time repetitions. We also apply the Close Limit Approximation
to investigate the effect of colliding black holes on surrounding scalar
fields. Notably, our results indicate that observables obtained through
perturbation theory may be extended to a significant segment of the merger
phase, where in principle only a numerical approach is appropriate.
