We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4 .
We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects .
The remaining 26 objects have trigonometric parallaxes from the literature .
We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel SED fitting algorithm to determine effective temperatures , bolometric luminosities , and radii .
Our uncertainties range from \sim 20K to \sim 150K in temperature , \sim 0.01 to \sim 0.06 in log ( L / L _ { \odot } ) and \sim 3 % to \sim 10 % in radius .
We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry .
We find evidence for the local minimum in the radius - temperature and radius - luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T _ { eff } \sim 2075 K , log ( L / L _ { \sun } ) \sim - 3.9 , and ( R / R _ { \sun } ) \sim 0.086 .
The existence of this local minimum is predicted by evolutionary models , but at temperatures \sim 400K cooler .
The minimum radius 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 in solar abundances accounts for the discrepancy between our findings and the evolutionary models .
We also report new color-absolute magnitude relations for optical and infrared colors useful for estimating photometric distances .
We study the optical variability of all 63 targets and find an overall variability fraction of 36 ^ { +9 } _ { -7 } % at a threshold of 15 milli-magnitudes in the I band , in agreement with previous studies .