Narrow band selected high redshift galaxy candidates contaminated by
  lower redshift O[III] ultrastrong emitter line galaxies
  Context. Lyman Break Galaxies (LBG) and Narrow Band (NB) surveys have been
successful at detecting large samples of high-redshift galaxies. Both methods
are subject to contamination from low-redshift interlopers. Aims. In this
paper, our aim is to investigate the nature of low-redshift interlopers in NB
Lyman-$\alpha$ emitters (LAE) searches. Methods. From previous HAWK-I NB
imaging at z $\sim$7.7 we identify three objects that would have been selected
as high-redshift LAEs had our optical data been one magnitude shallower (but
still one to two magnitudes fainter than the near infrared data). We follow-up
these objects in spectroscopy with XSHOOTER at the VLT. Results. Despite low
quality data due to bad weather conditions, for each of the three objects we
identify one, and only one emission line, in the spectra of the objects, that
we identify as the O[III]5007A line. This result combined to spectral energy
density fitting and tests based on line ratios of several populations of
galaxies 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 the
O[III] line appears to be a common source of contamination in high-redshift LBG
and LAE samples and we suggest that efforts be put to characterize with high
accuracy the O[III] luminosity function out to redshift $\sim$3 or higher.
