We study the production of a Higgs boson recoiling from a massless invisible system in e+e− collisions. This is a quite distinctive signature that can arise when the Higgs boson is produced in association with a massless dark photon, which can happen in BSM scenarios foreseeing an extra unbroken U(1) gauge group. Dark photons can indeed acquire effective couplings to the Higgs boson as occurs in models recently proposed to generate exponentially-spread Yukawa couplings. We analyze the signal and corresponding backgrounds for H→bb,, and estimate ILC and FCC-ee sensitivities in a model-independent way.
