Tests for the existence of horizon through gravitational waves from a
  small binary in the vicinity of a massive object
  In this letter we calculate the gravitational waves (GWs) emitted from a
small binary (SB) by solving the Teukolsky equation in the background of a
massive exotic compact object (ECO) which is phenomenologically described by a
Schwarzschild geometry with a reflective boundary condition at its "would-be"
horizon. The "continuous echo" waves propagating to infinity due to
reflectivity of ECO at its "would-be" horizon provide an exquisite probe to the
nature of the ECO's horizon.
