  Extreme mass ratio inspirals, in which a stellar-mass object merges with a
supermassive black hole, are prime sources for space-based gravitational wave
detectors because they will facilitate tests of strong gravity and probe the
spacetime around rotating compact objects. In the last few years of such
inspirals, the total phase is in the millions of radians and details of the
waveforms are sensitive to small perturbations. We show that one potentially
detectable perturbation is the presence of a second supermassive black hole
within a few tenths of a parsec. The acceleration produced by the perturber on
the extreme mass-ratio system produces a steady drift that causes the waveform
to deviate systematically from that of an isolated system. If the perturber is
a few tenths of a parsec from the extreme-mass ratio system (plausible in as
many as a few percent of cases) higher derivatives of motion might also be
detectable. In that case, the mass and distance of the perturber can be derived
independently, which would allow a new probe of merger dynamics.
