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We use deep Gemini/GMOS-S $g,r$ photometry to study the stellar populationsof the recently discovered Milky Way satellite candidates Horologium I, PictorI, Grus I, and Phoenix II. Horologium I is most likely an ultra-faint dwarfgalaxy at $D_\odot = 68\pm3$ kpc, with $r_h = 23^{+4}_{-3}$pc and $\langle$[Fe/H]$ \rangle = -2.40^{+0.10}_{-0.35}$\,dex. It's color-magnitude diagramshows evidence of a split sub-giant branch similar to that seen in someglobular clusters. Additionally, Gaia DR2 data suggests it is, or was, a memberof the Magellanic Cloud group. Pictor I with its compact size ($r_h =12.9^{+0.3}_{-0.2}$pc) and metal-poor stellar population ($\langle $[Fe/H]$\rangle = -2.28^{+0.30}_{-0.25}$) closely resembles confirmed star clusters.Grus I lacks a well-defined centre, but has two stellar concentrations withinthe reported half-light radius ($r_h = 1.77^{+0.85}_{-0.39}$ arcmin) and has amean metallicity of $\langle $[Fe/H]$ \rangle = -2.5\pm0.3$. Phoenix II has ahalf-light radius of $r_h = 12.6\pm2.5$pc and an $\langle $[Fe/H]$ \rangle =-2.10^{+0.25}_{-0.20}$ and exhibits S-shaped tidal arms extending from itscompact core. Great circles through each of these substructures intersect atthe Large Magellanic Cloud (LMC). This suggests that these objects are, or oncewere, satellites of the LMC.

On the Nature of Ultra-faint Dwarf Galaxy Candidates. III. Horologium I, Pictor I, Grus I, and Phoenix II