We report the discovery of an unusual emission line galaxy at redshift z = 1.68 found from near-IR imaging and spectroscopic observations with ISAAC/VLT of a faint gravitationally lensed R -band drop-out behind the lensing cluster Abell 1835 .
From the emission lines of [ O iii ] \lambda \lambda 4959,5007 , and H \beta this galaxy shows a moderate to high excitation and line equivalent widths typical of relatively metal-poor H ii galaxies .
Its apparent J magnitude translates to an absolute B -band magnitude M _ { B } \sim –16.4 once corrected for a gravitational magnification of 1.8 magnitudes .
This makes it one of the faintest intermediate redshift galaxies known so far . From the presence of emission lines and the available VRIJHK 
photometry we determine constraints on its dominant stellar population .
The only viable fit found is for a fairly young ( \sim 6–9 Myr ) burst suffering from a considerable extinction ( A _ { V } \sim 1.2–1.8 mag ) .
We find that this object resembles strongly H ii galaxies and intermediate redshift compact emission line galaxies , albeit suffering from a larger extinction than usual .
We also discuss the possible contamination introduced by such R -band drop-out galaxies in searches for z \ga 5 galaxies .