  Lortz et al. [Phys. Rev. B 73, 024512 (2006)] have utilized specific heat and
resistivity measurements as "thermal spectroscopies" to deconvolve the spectrum
of the electron-phonon interaction in YB$_6$ assuming a major role of the low
frequency phonon mode in mediating superconductivity. Here, we present direct
point-contact spectroscopy studies of the superconducting interaction in this
system. As a result the normalized superconducting gap reveals a strong
coupling with $2\Delta/k_BT_c = 4$ and moreover the spectra contain
nonlinearities typical of the electron-phonon interaction at energies around 8
meV. The in-magnetic-field measurements evidence that the phonon features found
in the second derivative of the current-voltage characteristics are due to the
energy dependence of the superconducting energy gap as their energy position
shrinks equally as the gap is closing. This is a direct proof that the
superconducting coupling in the system is due to the low energy Einstein-like
phonon mode associated with the yttrium ion vibrations in a perfect agreement
with determinations from bulk measurements.
