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We present the analyses of two short-timescale $(t_{\rm E} \sim 5~{\rmdays})$ microlensing events, KMT-2016-BLG-1820 and KMT-2016-BLG-2142. In bothlight curves, the brief anomalies were clearly captured and densely covered bythe Korea Microlensing Telescope Network survey. From these analyses, we findthat both events have small Einstein radii of $\theta_{\rm E} = 0.12~{\rmmas}$, suggesting that the binary-lens systems are composed of very low-masscomponents and/or are located much closer to the lensed stars than to Earth.From Bayesian analyses, we find that these binaries have total system masses of$0.043_{-0.018}^{+0.043}~M_{\odot}$ and $0.088_{-0.041}^{+0.120}~M_{\odot}$,implying that they are well within the very-low-mass regime. The estimatedlens-component masses indicate that the binary lenses consist of agiant-planet/brown-dwarf pair (KMT-2016-BLG-1820), and a dark/faint object pair(KMT-2016-BLG-2140) that are located near the deuterium-burning andhydrogen-burning mass limits, respectively. Both lens systems are likely to bein the Galactic disk with estimated distances of about $6$ kpc and $7$ kpc. Theprojected lens-components separations are $1.1$ AU and $0.8$ AU, and the massratios are $0.11$ and $0.20$. These prove that the microlensing method iseffective to identify these closely-separated very-low-mass binaries having lowmass-ratios.

KMT-2016-BLG-1820 and KMT-2016-BLG-2142: Two Microlensing Binaries Composed of Planetary-mass Companions and Very-Low-Mass Primaries