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We construct a Hertzsprung-Russell diagram for the stellar/substellarboundary based on a sample of 63 objects ranging in spectral type from M6V toL4. We report newly observed VRI photometry for all 63 objects and newtrigonometric parallaxes for 37 objects. The remaining 26 objects havetrigonometric parallaxes from the literature. We combine our optical photometryand trigonometric parallaxes with 2MASS and WISE photometry and employ a novelSED fitting algorithm to determine effective temperatures, bolometricluminosities, and radii. Our uncertainties range from ~20K to ~150K intemperature, ~0.01 to ~0.06 in log(L/Ls}) and 3% to 10% in radius. We check ourmethodology by comparing our calculated radii to radii directly measured vialong baseline optical interferometry. We find evidence for the local minimum inthe radius-temperature and radius-luminosity trends that signals the end of thestellar main sequence and the start of the brown dwarf sequence at T ~ 2075K,log(L/Ls)~ -3.9, and (R/Rs) ~ 0.086. The existence of this local minimum ispredicted by evolutionary models, but at temperatures ~400K cooler. The minimumradius happens near the locus of 2MASS J0523-1403, an L2.5 dwarf with V-K=9.42.We make qualitative arguments as to why the effects of the recent revision insolar abundances accounts for the discrepancy between our findings and theevolutionary models. We also report new color-absolute magnitude relations foroptical and infrared colors useful for estimating photometric distances. Westudy the optical variability of all 63 targets and find an overall variabilityfraction of 36^{+9}_{-7}% at a threshold of 15 milli-magnitudes in the I band,in agreement with previous studies.

The Solar Neighborhood XXXII. The Hydrogen Burning Limit