The STRong lensing Insights into the Dark Energy Survey (STRIDES)
  2017/2018 follow-up campaign: Discovery of 10 lensed quasars and 10 quasar
  pairs
  We report the results of the STRong lensing Insights from the Dark Energy
Survey (STRIDES) follow-up campaign of the late 2017/early 2018 season. We
obtained spectra of 65 lensed quasar candidates either with EFOSC2 on the NTT
or ESI on Keck, which confirm 10 new gravitationally lensed quasars and 10
quasar pairs with similar spectra, but which do not show a lensing galaxy in
DES images. Eight lensed quasars are doubly imaged with source redshifts
between 0.99 and 2.90, one is triply imaged by a group (DESJ0345-2545,
$z=1.68$), and one is quadruply imaged (quad: DESJ0053-2012, $z=3.8$). Singular
isothermal ellipsoid models for the doubles, based on high-resolution imaging
from SAMI on SOAR or NIRC2 on Keck, give total magnifications between 3.2 and
5.6, and Einstein radii between 0.49 and 1.97 arcseconds. After spectroscopic
follow-up, we extract multi-epoch $grizY$ photometry of confirmed lensed
quasars and contaminant quasar+star pairs from the first 4 years of DES data
using parametric multi-band modelling, and compare variability in each system's
components. By measuring the reduced ${\chi}^2$ associated with fitting all
epochs to the same magnitude, we find a simple cut on the less variable
component that retains all confirmed lensed quasars, while removing 94 per cent
of contaminant systems with stellar components. Based on our spectroscopic
follow-up, this variability information can improve selection of lensed quasars
and quasar pairs from 34-45 per cent to 51-70 per cent, with the majority of
remaining contaminants being compact star-forming galaxies. Using mock lensed
quasar lightcurves we demonstrate that selection based only on variability will
over-represent the quad fraction by 10 per cent over a complete DES
magnitude-limited sample (excluding microlensing differences), explained by the
magnification bias and hence lower luminosity (more variable) sources in quads.
