OGLE-2018-BLG-0532Lb: Cold neptune with possible jovian sibling

We report the discovery of the planet OGLE-2018-BLG-0532Lb, with very obvious signatures in the light curve that lead to an estimate of the planet-host mass ratio q = Mplanet Mhost ≃ 1 ´ 10-4. Although there are no obvious systematic residuals to this double-lens/single-source (2L1S) fit, we find that χ2 can be significantly improved by adding either a third lens (3L1S, Δχ2 = 81) or second source (2L2S, Δχ2 = 77) to the lens-source geometry. After thorough investigation, we conclude that we cannot decisively distinguish between these two scenarios and therefore focus on the robustly detected planet. However, given the possible presence of a second planet, we investigate to what degree and with what probability such additional planets may affect seemingly single-planet light curves. Our best estimates for the properties of the lens star and the secure planet are a host mass M ∼ 0.25 Me, system distance DL ∼ 1 kpc, and planet mass mp,1 = 8 MÅ with projected separation a1^ = 1.4 au. However, there is a relatively bright I = 18.6 (and also relatively blue) star projected within lens, and if future high-resolution images show that this is coincident with the lens, then it is possible that it is the lens, in which case, the lens would be both more massive and more distant than the best-estimated values above