  Improvements to dual-comb interferometers will benefit precision spectroscopy
and sensing, distance metrology, tomography, telecommunications etc. A specific
requirement of such interferometers is to enforce mutual coherence between the
two combs over the measurement time. With feed-forward relative stabilization
of the carrier-enveloppe offset frequencies, we experimentally realize such
mutual coherence over times that exceed 300 seconds, two orders of magnitude
longer than state-of-the-art systems. Illustration is given with near-infrared
Fourier transform molecular spectroscopy, where two combs of slightly different
repetition frequencies replace a scanning two-beam interferometer. Our
technique can be implemented with any frequency comb generators including
microresonators or quantum cascade lasers.
