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We report the results of the STRong lensing Insights from the Dark EnergySurvey (STRIDES) follow-up campaign of the late 2017/early 2018 season. Weobtained spectra of 65 lensed quasar candidates either with EFOSC2 on the NTTor ESI on Keck, which confirm 10 new gravitationally lensed quasars and 10quasar pairs with similar spectra, but which do not show a lensing galaxy inDES images. Eight lensed quasars are doubly imaged with source redshiftsbetween 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, $z=3.8$). Singularisothermal ellipsoid models for the doubles, based on high-resolution imagingfrom SAMI on SOAR or NIRC2 on Keck, give total magnifications between 3.2 and5.6, and Einstein radii between 0.49 and 1.97 arcseconds. After spectroscopicfollow-up, we extract multi-epoch $grizY$ photometry of confirmed lensedquasars and contaminant quasar+star pairs from the first 4 years of DES datausing parametric multi-band modelling, and compare variability in each system'scomponents. By measuring the reduced ${\chi}^2$ associated with fitting allepochs to the same magnitude, we find a simple cut on the less variablecomponent that retains all confirmed lensed quasars, while removing 94 per centof contaminant systems with stellar components. Based on our spectroscopicfollow-up, this variability information can improve selection of lensed quasarsand quasar pairs from 34-45 per cent to 51-70 per cent, with the majority ofremaining contaminants being compact star-forming galaxies. Using mock lensedquasar lightcurves we demonstrate that selection based only on variability willover-represent the quad fraction by 10 per cent over a complete DESmagnitude-limited sample (excluding microlensing differences), explained by themagnification bias and hence lower luminosity (more variable) sources in quads.

The STRong lensing Insights into the Dark Energy Survey (STRIDES) 2017/2018 follow-up campaign: Discovery of 10 lensed quasars and 10 quasar pairs