  We report the LBT/MODS1 spectroscopic confirmation of two images of faint
Lyman alpha emitters at $z=6.4$ behind the Frontier Fields galaxy cluster
MACSJ0717.5+3745. A wide range of lens models suggests that the two images are
highly magnified, with a strong lower limit of mu>5. These are the faintest z>6
candidates spectroscopically confirmed to date. These may be also multiple
images of the same z=6.4 source as supported by their similar intrinsic
properties, but the lens models are inconclusive regarding this interpretation.
To be cautious, we derive the physical properties of each image individually.
Thanks to the high magnification, the observed near-infrared (restframe
ultraviolet) part of the spectral energy distributions and Ly-alpha lines are
well detected with S/N(m_1500)>~10 and S/N(Ly-alpha)~10-15. Adopting mu>5, the
absolute magnitudes, M_1500, and Ly-alpha fluxes, are fainter than -18.7 and
2.8x10^(-18)erg/s/cm2, respectively. We find a very steep ultraviolet spectral
slope beta=-3.0+/-0.5 (F_lambda=lambda^(beta)), implying that these are very
young, dust-free and low metallicity objects, made of standard stellar
populations or even extremely metal poor stars (age<~30Myr, E(B-V)=0 and
metallicity 0.0-0.2 Z/Zsolar). The objects are compact (< 1 kpc^(2)), and with
a stellar mass M* < 10^(8) M_solar. The very steep beta, the presence of the
Ly-alpha line and the intrinsic FWHM (<300 km/s) of these newborn objects do
not exclude a possible leakage of ionizing radiation. We discuss the
possibility that such faint galaxies may resemble those responsible for cosmic
reionization.
