  We explore a unique electromagnetic signature of stellar-mass compact-object
binaries long before they are detectable in gravitational waves. We show that
gravitational lensing of light emitting components of a compact-object binary,
by the other binary component, could be detectable in the nearby universe. This
periodic lensing signature could be detected from present and future X-ray
observations, identifying the progenitors of binaries that merge in the LIGO
band, and also unveiling populations that do not merge, thus providing a tracer
of the compact-object binary population in an enigmatic portion of its life. We
argue that periodically repeating lensing flares could be observed for ~100
ksec orbital-period binaries with the future Lynx X-ray mission, possibly
concurrent with gravitational wave emission in the LISA band. Binaries with
longer orbital periods could be more common and be detectable as single lensing
flares, though with reliance on a model for the flare that can be tested by
observations of succeeding flares. Non-detection of such events, even with
existing X-ray observations, will help to constrain the population of EM bright
compact-object binaries.
