  Interferometry is an indispensable tool across all the natural sciences.
Recently, a new type of interferometer based on phase-sensitive Fano resonances
has been proposed and implemented. In these Fano interferometers, the two arms
are formed by a spectrally broad continuum channel, and a spectrally narrow
resonant bound state scattering channel, respectively. We show that the
textbook relation between interference visibility and coherence known from
double-slit- or Mach-Zehnder-interferometers does not apply to Fano
interferometers, because the physical origin of the interference extrema is
different. We then show how instead the asymmetry of Fano spectra can be
exploited to quantify coherence in Fano interferometers.
