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tokens: for EW 0 (Ly α ) and β in A370-L57, and the gray areas represent the 1 σ confidence interval. In the right panel, the same information is represented by the filled star with error bars. The dependence of β on the age and metallicity of the population (middle panel) is more complex. At very young age ( Myr), β is almost independent of t and higher at lower metallicity (due to increased nebular emission). However, as the population ages and the nebular emission decreases, β increases in the Z > 0 populations but decreases for PopIII. By t ∼ 10 Myr the nebular emission has become negligible and β is monotonously higher for higher metallicity. The observed value for β in A370-L57 is consistent with almost any metallicity at some age. However, since t > 3 Myr is ruled out for Z > 4 × 10 −4 by EW 0 (Ly α ), the observed β requires Z ≲ 4 × 10 −4 . The right panel rules out a PopIII-dominated SED for A370-L57, because for the observed β the required EW 0 (Ly α ) would be ∼1000 Å. Despite this, our single population delayed exponential model is likely a gross oversimplification of the actual SFH of the galaxy. While the currently available data do not allow us to test more complex SFHs, a wider range of stellar ages and metallicities may be present. If older or more metallic stars contribute significantly to the UV continuum emission, younger and/or less metallic ones are required to compensate for their lower production rate of LyC photons in order to reproduce the observed Ly α luminosity. A conclusive diagnostic of the source of the Ly α emission in A370-L57 requires the determination of UV emission line ratios. Recently, Vanzella et al. ( 2016 ) measured line ratios C iv /Ly α = 0.25, He ii /Ly α = 0.067, and O iii ]/Ly α = 0.16 in the spectrum of a lensed z = 3.11 LAE with comparably low luminosity ( M UV = −17.0). If A370-L57 has similar line ratios, the C iv λ 1550, He ii λ 1640, and O iii ] λλ 1660, 1666 lines, among others, would be detectable with deep near-IR spectroscopy on current 10 m class telescopes. 9. Conclusions In this work we have taken advantage of strong lensing by the Abell 370 galaxy cluster in combination with deep imaging from the Hubble Frontier Fields and SHARDS Frontier Fields programs and MUSE spectroscopy to perform a detailed analysis of an Ly α emitter. This source would be fainter than magnitude 30 in the continuum if it were not magnified by the gravitational lens. The source, A370-L57, is a z = 5.75 galaxy with a low Ly α luminosity ( L (Ly α ) ∼ 10 42 erg s −1 ), which, however, implies an extreme Ly α EW of ∼420 Å. This value of the EW is exceptional among known galaxies in this luminosity range and redshift. The even fainter source B2, with similar properties and confirmed spectroscopically to be at the same redshift, is just ∼1 kpc away in projection and could be another star-forming region in the same galaxy or a close neighbor. The physical properties of A370-L57 are similar to those of galaxies with