  We study a novel electromagnetic signature of supermassive black hole
binaries whose inspiral starts being dominated by gravitational wave (GW)
emission. Recent simulations suggest that the binary's member BHs can continue
to accrete gas from the circumbinary accretion disk in this phase of the
binary's evolution, all the way until coalescence. If one of the binary members
produces a radio jet as a result of accretion, the jet precesses along a
biconical surface due to the binary's orbital motion. When the binary enters
the GW phase of its evolution, the opening angle widens, the jet exhibits
milliarcsecond scale wiggles, and the conical surface of jet precession is
twisted due to apparant superluminal motion. The rapidly increasing orbital
velocity of the binary gives the jet an appearance of a "chirp." This helical
chirping morphology of the jet can be used to infer the binary parameters. For
binaries with mass 10^7--10^10 Msun at redshifts z<0.5, monitoring these
features in current and archival data will place a lower limit on sources that
could be detected by eLISA and Pulsar Timing Arrays. In the future,
microarcsecond interferometry with the Square Kilometer Array will increase the
potential usefulness of this technique.
