WHT follow-up observations of extremely metal-poor stars identified from
  SDSS and LAMOST
  We have identified several tens of extremely metal-poor star candidates from
SDSS and LAMOST, which we follow-up with the 4.2m WHT telescope to confirm
their metallicity.We follow a robust two-step methodology. We first analyze the
SDSS and LAMOST spectra. A first set of stellar parameters is derived from
these spectra with the FERRE code, taking advantage of the continuum shape to
determine the atmospheric parameters, in particular, the effective temperature.
Second, we select interesting targets for follow-up observations, some of them
with very low-quality SDSS or LAMOST data. We then obtain and analyze
higher-quality medium-resolution spectra obtained with ISIS on the WHT
telescope to arrive at a second, more reliable, set of atmospheric parameters.
This allows us to derive, with accuracy, the metallicity, and confirm the
extremely metal-poor nature in most cases. In this second step we also employ
FERRE, but we take a running mean to normalize both the observed and the
synthetic spectra, and therefore the final parameters do not rely on having an
accurate flux calibration or continuum placement. In order to verify our
results we have analyzed with the same tools, and following the same procedure,
six well-known metal-poor stars, five of them at [Fe/H]<-4, showing that our
methodology is able to get accurate metallicity determinations down to
[Fe/H]<-5.0. The results for these six reference stars give us confidence on
the metallicity scale for the rest of the sample. In addition, we present 12
new extremely metal-poor candidates: two stars at [Fe/H]=-4, six more in the
range -4<[Fe/H]<-3.5, and four more at -3.5<[Fe/H]<-3.0.
