LBT/MODS1 spectroscopy of two faint sources at z=6.4

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...