The Pristine survey III: Spectroscopic confirmation of an efficient
  search for extremely metal-poor stars
  The Pristine survey is a narrow-band, photometric survey focused around the
wavelength region of the Ca II H & K absorption lines, designed to efficiently
search for extremely metal-poor stars. In this work, we use the first results
of a medium-resolution spectroscopic follow-up to refine the selection criteria
for finding extremely metal-poor stars ($\textrm{[Fe/H]} \leq -3.0$) in the
Pristine survey. We consider methods by which stars can be selected from
available broad-band and infrared photometry plus the additional Pristine
narrow-band photometry. The spectroscopic sample presented in this paper
consists of 205 stars in the magnitude range $14 < V < 18$. Applying the
photometric selection criteria cuts the sample down to 149 stars, and from
these we report a success rate of 70% for finding stars with $\textrm{[Fe/H]}
\leq -2.5$ and 22% for finding stars with $\textrm{[Fe/H]} \leq -3.0$. These
statistics compare favourably with other surveys that search for extremely
metal-poor stars, namely an improvement by a factor of $\sim 4-5$ for
recovering stars with $\textrm{[Fe/H]} \leq -3.0$. In addition, Pristine covers
a fainter magnitude range than its predecessors, and can thus probe deeper into
the Galactic halo.
