Cheongho Han

To search for very wide-orbit binary-lens events, we crossmatch microlensing events detected by the Optical Gravitational Lensing Experiment (OGLE)-IV and KMTNet between 2011 and 2024. After removing contaminants from variable stars, we identify OGLE-2016-BLG-0523 as a microlensing event showing two short-lived bumps in its light curve, with the two separated by 8.36 yr. We investigate three scenarios to explain the two bumps, including a binary-lens single-source (2L1S), a single-lens binary-source (1L2S), and two unrelated single-lens single-source events (2×1L1S). The 1L2S interpretation is excluded due to its poorer fit to the data, while the 2×1L1S scenario is disfavored by the low event rate, unphysical total source flux, and unphysical color for one of the two putative sources. For the surviving 2L1S interpretation, the Einstein timescales of the two bumps are 8.343 ± 0.076 days and 3.231 ± 0.047 days, respectively, implying a binary mass ratio of q = 0.15. A Bayesian analysis indicates that the primary lens is most likely a low-mass M dwarf, while the companion is probably a low-mass brown dwarf or super-Jupiter. The inferred projected separation is about 450 au, making the companion the widest low-mass object discovered via microlensing to date. Our discovery highlights both the promise and challenges of identifying very wide, low-mass companions through microlensing. Although upcoming facilities such as Roman and Earth 2.0 space telescopes will improve sensitivity to such events, overcoming degeneracies and distinguishing them from variable phenomena will remain essential for building a robust statistical sample.

We present a comprehensive photometric and spectroscopic analysis of the contact eclipsing binary BS Cas, based on 22 yr multisite light curves and 2 yr high-resolution spectra. The long-term photometry shows pronounced year-to-year changes in the light-curve shape, including eclipse-depth reversals between the primary and secondary minima and variations in the O’Connell effect. The first double-lined radial-velocity (RV) curves of BS Cas are derived from the spectra using broadening-function (BF) profiles. Sharp peaks absent from the 2018 BFs appear in the 2020 BFs, suggesting an additional component along the same line of sight as BS Cas. Simultaneous modeling of the BV R light curves and the RVs shows that BS Cas is a contact binary in which the more massive component is cooler than its less massive companion, with a mass ratio of 2.515 and a fill-out factor of 32.3%. Multiepoch modeling suggests that the long-term photometric variations arise from evolving starspots and changes in the third-light contribution. The eclipse-timing variation diagram confirms a secular orbital period decrease at a rate of −2.305 × 10 ⁻⁷ days yr ⁻¹ consistent with angular momentum loss and mass transfer, along with an 18.376 yr modulation interpreted as a light-travel-time effect from a third body. The TESS timings reveal anticorrelated variations between the primary and secondary minima that appear to result from differential rotation of high-latitude starspots. These results suggest that BS Cas is a physically complex contact binary in which long-term photometric and eclipse-timing variations reflect multiple processes, including magnetic activity and the influence of a third body.

The Systematic KMTNet Planetary Anomaly Search series was conducted using the KMTNet data archived from 2016–2019. From this first phase of the series, we reported a total of 50 planetary systems hidden in the data archive, which represent about 35% of the total microlensing planets discovered from 2016–2019, demonstrating that this semi-machine-based search is a crucial channel for building a complete microlensing planet sample. We continue this series for 2021 and beyond to expand the microlensing planet sample. In this work for the 2021 KMTNet high-cadence fields (Prime fields), we find seven hidden planetary systems and three planet candidates. These new planets represent about 33% of the total microlensing planets discovered within the Prime fields observed during the 2021 bulge season. While the by-eye search is the primary channel for detecting microlensing planets (i.e., two-thirds of microlensing planet discoveries), this work clearly shows that a systematic search series is still necessary for constructing a complete microlensing planet sample. Such a sample is essential for conducting unbiased statistical studies of planet demographics in our Galaxy. Datasets for all of the events used for analyses in this work are publicly available: doi: https://zenodo.org/records/21472225 .

We present the analysis of the microlensing event KMT-2026-BLG-0083, which was independently detected by the KMTNet, OGLE, and PRIME surveys and also monitored by the DREAMS survey. The combined data provide dense coverage of two distinct short-duration anomalies in the light curve that cannot be reproduced by a standard binary-lens model. Independent analyses of the two anomalies indicate that each is produced by a planetary companion to the lens, motivating a triple-lens interpretation. The modeling yields two pairs of degenerate solutions arising from the well-known inner–outer degeneracy, with each pair exhibiting two local solutions depending on whether the source passes above or below the distant planetary companion. The preferred model indicates two giant planets with mass ratios q ₂ = (5.25 ± 0.08) × 10 ⁻³ and q ₃ = (3.04 ± 0.24) × 10 ⁻³ orbiting a common host. Bayesian analysis indicates that the host is an M-dwarf star with a mass of 0.4 8 − 0.28 + 0.36 M ⊙ , hosting two giant planets with masses of 2.6 5 − 1.55 + 1.96 M J and 1.5 4 − 0.90 + 1.14 M J . The projected planet–host separations are 10 . 6 − 2.2 + 1.7 au and 1 . 7 − 0.3 + 0.3 au, placing the inner planet near the host’s snow line and the outer planet at a substantially larger separation. Thus, KMT-2026-BLG-0083L becomes the seventh confirmed multiple-planet system discovered through gravitational microlensing, providing another example of a cold giant planetary system orbiting a subsolar-mass star.

We report the discovery of four cold giant planets identified through the analysis of microlensing events detected by high-cadence surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. The planetary signals appear as short-duration anomalies in the light curves and are well described by binary-lens single-source models with mass ratios between the lens components of order q ∼ 10 ⁻³ . Finite-source effects are securely measured in three out of four events, enabling determinations of the angular Einstein radius. A Bayesian analysis incorporating the measured event timescale and angular Einstein radius yields host masses of ∼0.07–0.6 M _⊙ and companion masses of ∼0.2–2.5 M _Jup , confirming that all companions lie in the giant-planet regime. The projected separations are ∼0.7–6 au, placing all planets at or beyond the snow lines of their host stars. The inferred lens distances span ∼6.6–7.9 kpc, with all systems consistent with bulge lenses. These detections expand the sample of cold giant planets from homogeneous high-cadence surveys and highlight the sensitivity of microlensing to planetary systems beyond the snow line, providing further constraints on the occurrence and properties of giant planets around low-mass stars.

We present observations and analysis of two low planet/host mass-ratio ( q ) microlensing planets discovered in high-magnification events. KMT-2025-BLG-0811Lb has q ∼ 4.5 × 10 ⁻⁵ , and a Bayesian analysis favors a super-Earth/mini-Neptune orbiting an M- or K-dwarf host at a projected separation of ∼3 au. KMT-2025-BLG-0912Lb has q = 2.6 × 10 ⁻⁴ and likely hosts a super-Earth/mini-Neptune around either a low-mass M dwarf or a brown dwarf at ∼1 au. Even with an observing cadence of Γ > 30 hr ⁻¹ during the planetary signal, KMT-2025-BLG-0811 still exhibits the “central-resonant” degeneracy. Reviewing nine such events, we find that the “central-resonant” degeneracy can be divided into two distinct types that occupy separate regions in the plane of q and normalized source radius ( ρ ). Type I events have similar q but substantially different ρ and are more difficult to resolve from the light curves. For Type II events, the “resonant” solutions have relatively lower q and larger ρ . Our review provides guidance for searching for the alternative solution once one solution has been identified.

We present detailed analyses of three anomalous microlensing events—KMT-2021-BLG-0209, KMT-2021-BLG-0901, and OGLE-2025-BLG-0356—identified from a systematic reexamination of KMTNet light curves for which previous modeling attempts failed or left persistent residuals. Although all three events show caustic-related features consistent with binary-lens microlensing, we find that their full light-curve structures can be described by four-body configurations that required four-body configurations involving a binary lens and a binary source (2L2S). In KMT-2021-BLG-0209, weak caustic-exit residuals arise from a faint companion source undergoing an additional caustic interaction. In KMT-2021-BLG-0901, a late-time rebrightening is produced when the secondary source encounters the resonant caustic long after the primary. For OGLE-2025-BLG-0356, we test the degeneracy between system is described by a triple-lens single-source (3L1S) and 2L2S interpretations of a short isolated anomaly and find that the 2L2S model provides a significantly better fit. Source colors and magnitudes indicate binary sources composed of (G8V, M3V), (G8V, K2V), and (G6V, G8V) stars for the three events, respectively. Bayesian inference suggests that the lenses are predominantly low-mass binaries, including one system (KMT-2021-BLG-0901) with a companion consistent with a brown dwarf. These events add to the growing sample of well-characterized 2L2S systems and underscore the importance of systematically testing complex models, particularly in anticipation of the high-precision microlensing data expected from the Roman Space Telescope survey.

To expand the homogeneous microlensing planetary sample of the Korea Microlensing Telescope Network (KMTNet), we investigate six planetary candidates identified by the AnomalyFinder search in the 2023 prime-field data, namely KMT-2023-BLG-1592, OGLE-2023-BLG-0766, KMT-2023-BLG-0332, KMT-2023-BLG-0486, KMT-2023-BLG-0792, and OGLE-2023-BLG-1043. Light-curve modeling indicates that the first two events have planetary mass ratios of log q ∼ − 3.0 and −2.6, while the third exhibits a brown dwarf mass ratio of log q ∼ − 1.4 . The remaining three events show the well-known degeneracy between the binary-lens single-source (2L1S) and single-lens binary-source (1L2S) interpretations. A Bayesian analysis yields companion masses of about 0.6 and 1.2 Jupiter masses for the two planetary systems, likely orbiting beyond the snow lines of M- or K-dwarf hosts. A review of the KMTNet planetary sample shows that candidates discovered by AnomalyFinder are significantly more likely to exhibit the 2L1S/1L2S degeneracy, consistent with the tendency of AnomalyFinder to detect subtler planetary signals.

We present detailed light-curve analyses of 10 binary-lens microlensing events observed during the 2023–2025 seasons and selected as candidates for hosting brown dwarf companions. The sample includes OGLE-2023-BLG-0249, KMT-2023-BLG-1246, OGLE-2023-BLG-0079, KMT-2024-BLG-0072, KMT-2024-BLG-0897, KMT-2024-BLG-1876, KMT-2024-BLG-2379, KMT-2025-BLG-0922, KMT-2025-BLG-1056, and KMT-2025-BLG-2427. For each event, we carry out modeling of the light curve, explore relevant degeneracies, and, when finite-source effects are present, determine the angular Einstein radius. For OGLE-2023-BLG-0249, we additionally measure the microlens parallax, which allows a direct determination of the lens masses and distance. For the remaining events, we estimate the physical lens properties via Bayesian analyses incorporating Galactic priors. The resulting posteriors show that the lens companions in all systems have median masses in the brown dwarf regime, and the lenses of two events (KMT-2025-BLG-0922 and KMT-2025-BLG-1056) are consistent with binaries in which both lens components fall within the brown dwarf mass range. Spanning a wide range of projected separations and distances, these detections illustrate the power of high-cadence microlensing surveys to build a census of brown dwarf companions, including faint and distant systems beyond the reach of flux-limited methods.

We initiate the systematic search for planets in the 2023 data of the Korea Microlensing Telescope Network (KMTNet), focusing on those planets found by the KMTNet AnomalyFinder with low preliminary estimates of the mass ratio, q < 2 × 10 ⁻⁴ . The 2023 season is the first for which the photometry of all events was re-reduced prior to the AnomalyFinder search, potentially increasing its sensitivity to planets. We find three strong low- q planet candidates, KMT-2023-BLG-0164 ( q ∼ 1.3 × 10 ⁻⁴ ), KMT-2023-BLG-1286 ( q ∼ 1.9 × 10 ⁻⁴ ), and KMT-2023-BLG-1746 ( q ∼ 8 × 10 ⁻⁵ ). KMT-2023-BLG-0164 is notable in that the source is projected on a very bright ( I = 16.0) foreground star, which is either the planet’s host or (more likely) a companion to the host. We obtain a spectrum, finding that its mass and distance are M ∼ 1.0 M _⊙ and D ∼ 1.5 kpc, respectively, the latter being the distance of the lens ( D _L ) regardless of whether the spectroscopic target is the host or its companion. We also analyze two other candidates, KMT-2023-BLG-0614 and KMT-2023-BLG-1593, which are unlikely to enter the statistical sample owing to their ambiguous interpretations as possible nonplanetary events.