KMT-2022-BLG-1818Lb,c: A Cold Super-Jupiter with a Saturn Sibling

We present the discovery and analysis of the sixth microlensing two-planet system, KMT-2022-BLG-1818Lb,c, detected by a follow-up program targeting high-magnification events. The second planet was identified from implausible physical parameters for the models with a single planet. Both planets are subject to the well-known "close/wide" degeneracy, although for the first planet, which has a super-Jovian mass ratio of q2 ≃ 5 × 10−3 in both solutions, the close topology, with a normalized separation of s ≃ 0.70, is clearly preferred by ∆χ2 = 26. However, contrary to all previous two-planet microlensing systems, the mass ratio for the second planet, q3, is substantially (factor of ∼10) different for the Close and Wide topologies of the first planet. While this degeneracy is resolved in the present case due to high-cadence follow-up observations, the appearance of this new degeneracy indicates the need for caution in the analysis of future two-planet systems. A Bayesian analysis suggests that the host is likely a K dwarf star in the Galactic disk. The first planet is probably a super-Jupiter on a Jupiter-like orbit, while the second planet is a Saturn-class planet on either a Mercury-like or Saturn-like orbit.