  A precise electromagnetic measurement of the sky coordinates and redshift of
a coalescing black hole binary holds the key for using its gravitational wave
(GW) signal to constrain cosmological parameters and to test general
relativity. Here we show that the merger of ~10^{6-7}M_sun black holes is
generically followed over a period of years by multiple electromagnetic flares
from tidally disrupted stars. The sudden recoil imparted to the merged black
hole by GW emission promptly fills its loss cone and results in a tidal
disruption rate of stars as high as ~0.1 per year. The prompt disruption of a
star within a single galaxy over a short period provides a unique
electromagnetic flag of a recent black hole coalescence event, and sequential
disruptions could be used on their own to calibrate the expected rate of GW
sources for pulsar timing arrays or the proposed Laser Interferometer Space
Antenna (LISA).
