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Context. Lyman Break Galaxies (LBG) and Narrow Band (NB) surveys have beensuccessful at detecting large samples of high-redshift galaxies. Both methodsare subject to contamination from low-redshift interlopers. Aims. In thispaper, our aim is to investigate the nature of low-redshift interlopers in NBLyman-$\alpha$ emitters (LAE) searches. Methods. From previous HAWK-I NBimaging at z $\sim$7.7 we identify three objects that would have been selectedas high-redshift LAEs had our optical data been one magnitude shallower (butstill one to two magnitudes fainter than the near infrared data). We follow-upthese objects in spectroscopy with XSHOOTER at the VLT. Results. Despite lowquality data due to bad weather conditions, for each of the three objects weidentify one, and only one emission line, in the spectra of the objects, thatwe identify as the O[III]5007A line. This result combined to spectral energydensity fitting and tests based on line ratios of several populations ofgalaxies we infer that the 3 objects are ultrastrong line emitters at redshifts$\sim$1.1. Conclusions. From this work and the literature we remark that theO[III] line appears to be a common source of contamination in high-redshift LBGand LAE samples and we suggest that efforts be put to characterize with highaccuracy the O[III] luminosity function out to redshift $\sim$3 or higher.

Narrow band selected high redshift galaxy candidates contaminated by lower redshift O[III] ultrastrong emitter line galaxies