GJ 1001BC          L4.5         ID  1 Is a binary L dwarf with nearly equal luminosity components (Golimowski et al., 2004AJ....128.1733G). Golimowski et al. (2006AAS...208.0504G, 2007IAUS..240..329G) report a preliminary total system dynamical mass of 0.10M_{sun}_ based on orbital mapping using HST and Very Large Telescope. The conservative assumption of a mass ratio >=3:2 based on nearly equal luminosity would make individual masses range between 0.04M_{sun}_ and 0.06M_{sun}_, thus placing both objects in the brown dwarf regime. We derive T_eff_=1725+/-21 and log(L/L_{sun}_)=-4.049+/-0.48 for each component, assuming the two objects are identical. These numbers are generally above the hydrogen burning limit numbers predicted by models but below our numbers (Table 8). This inconsistency is further evidence that the hydrogen burning limit must happen at higher luminosities and temperatures than what is predicted by the currently accepted models.
LEHPM1-0494 A      M6.0V        ID  3 And LEHPM1-0494 B are reported by Caballero (2007A&A...462L..61C) to be a wide common proper motion binary with separation of 78''. We report trigonometric parallaxes for both components based on individual reductions of the same field of view, and derive distances of 26.88^+1.51^_1.36_pc for the A component and 25.14^+1.40^_-1.26_pc for the B component for a projected separation of ~2100AU. These trigonometric distances are in good agreement with Caballero's distance estimate of 23+/-2pc and support his claim of a physical association between these two objects.
LEHPM1-0494 B      M6.0V        ID  2 And LEHPM1-0494 A are reported by Caballero (2007A&A...462L..61C) to be a wide common proper motion binary with separation of 78''. We report trigonometric parallaxes for both components based on individual reductions of the same field of view, and derive distances of 26.88^+1.51^_1.36_pc for the A component and 25.14^+1.40^_-1.26_pc for the B component for a projected separation of ~2100AU. These trigonometric distances are in good agreement with Caballero's distance estimate of 23+/-2pc and support his claim of a physical association between these two objects.
LHS 1604           M7.5V        ID 12 Was first reported by Cruz et al. 2007 (cat. J/AJ/133/439) as being over-luminous by ~0.6mag in J. They suggested that the near-infrared photometry is consistent with an unresolved M7.5V/M9.0V binary. LHS 1604 is the only star in our sample for which we were not able to calculate T_eff_ or perform an SED fit using the procedures outlined in Section 5--the fits diverged due to a large infrared excess. We observed LHS 1604 using high-resolution laser guide star adaptive optics on Gemini North and preliminary results do not show a resolved companion. We defer a thorough analysis of this target to a future publication where we discuss our high-resolution observations and use them to place limits on the properties of the putative companion (S. B. Dieterich et al., in preparation). We are also monitoring LHS 1604 for astrometric perturbations but it is too early to notice any trends.
2MASS J0451-3402   L0.5         ID 15 Has the highest photometric variability in our sample. It was first noted as a photometrically variable target by Koen (2004MNRAS.354..378K), who reported a sinusoidal trend with a period of 3.454 days and mean amplitude of ~1% (10mmag), though varying to as high as 4% (40mmag). While our observations do not have the cadence necessary to obtain phase information, the variability of 51mmag in the I band we detect is in agreement, if not somewhat higher, to that of Koen (2004MNRAS.354..378K). It is interesting to note the spike in variability around T_eff_~2100K in Figure 9. Further investigation is needed to determine whether this trend has a physical cause associated with that temperature range or whether this is a coincidence.
2MASS J0523-1403   L2.5         ID 17 Is discussed throughout this paper as the object closest to the local minimum in the luminosity-radius and temperature-radius trends (Figure 11). As we discussed in Section 7, there is strong evidence indicating that the end of the stellar main sequence must lie in its proximity in parameter space. The target has been described as having variable radio and H{alpha} emission (Berger, 2002ApJ...572..503B; Antonova et al., 2007A&A...472..257A; Berger et al., 2010ApJ...709..332B). Despite the common association between H{alpha} emission and youth, we note that it is difficult to conceive of a target with such a small radius (R/R_{sun}_=0.086+/-.0031) as being young. As discussed in Section 6.5, radio emission is often used as a probe of magnetic fields and may be accompanied by optical variability if they result in auroral phenomena. We detect no significant I band variability for 2MASS J0523-1403 (upper limit ~11.7mmag), meaning that either the star was in a mostly quiescent state during the ~3yr for which we monitored the target (2010.98-2013.12) or that the link between radio emission and I band variability is not universal.
SSSPM J0829-1309   L1.0         ID 23 Is an object very similar to 2MASS J0523-1403 but slightly more luminous. The two objects have 1{sigma} uncertainties that overlap in radius and T_eff_, but not luminosity. As shown in Figure 11, the location of SSSPM J0829-1309 is crucial for establishing 2MASS J0523-1403 as being close to the minimum of the radius trends. Taken together, 2MASS J0523-1403 and SSSPM J0829-1309 show that the radius trends in Figure 11 are real, and therefore, the conclusions we draw in this paper are not the result of one isolated odd object (i.e., 2MASS J0523-1403).
LHS 2397aAB        M8.5V (joint ID 35 Is an M8.0V/L7.5 (infrared spectral type for secondary) binary (Freed et al., 2003ApJ...584..453F). Dupuy et al. (2009ApJ...699..168D) report a total system dynamical mass of 0.146^+0.015^_-0.013_M_{sun}_. Konopacky et al. (2010ApJ...711.1087K) derive individual dynamical masses of 0.09+/-0.06M_{sun}_ for the primary and 0.06+/-0.05M_{sun}_ for the secondary. The system is therefore an important probe of the hydrogen burning mass limit because two coeval components presumably with the same metallicity lie on opposite sides of the stellar/substellar boundary. We are mapping the astrometric orbit for this system in a manner similar to that discussed in Section 6.6 for DENIS J1454-6604AB and will publish refined individual dynamical masses as soon as orbital mapping is complete.
LEHPM2-0174        M6.5V        ID 40 Appears over-luminous in Figure 4. It is most likely an unresolved multiple, a young object, or both. We note that we could not determine a reliable source for the spectral type of this object, thus leaving open the possibility that it has been miss-characterized as an M6.5V. LEHPM2-0174 is excluded from Figure 11 because scaling the figure to fit its radius (0.173R_{sun}_) would make the figure difficult to read.
Kelu-1AB           L2.0 (joint) ID 41 Is a well-known L2/L4 binary (Liu & Leggett, 2005ApJ...634..616L). That study notes that the presence of Li{lambda}6708 makes both components substellar with masses {lesssim}0.06M_{sun}_ according to the lithium test of Rebolo et al. (1992ApJ...389L..83R), although they note that the Li{lambda}6708 detection is tenuous. Deconvolution of this system would provide important information about the hydrogen burning limit due to its location in the temperature-radius trend (Figure 11(b)). If we assume that the system is an equal luminosity binary, then the deconvolved radii of the components are ~0.089M_{sun}_. That number would further constrain the position of 2MASS J0523-1309 as being in the minimum of the radius trend. However, because the components of Kelu1-AB do not have equal luminosities, we can expect the A component to be a more massive brown dwarf or a stellar component with mass just above the hydrogen burning limit. In either case, the A component would have a smaller radius than the B component. Determining the precise radius, T_eff_ and luminosity of the A component is crucial for determining the exact location of the point of minimal radius in Figure 11.
2MASS J1705-0516AB L0.5 (joint) ID 56 Was first reported as an M9V/L3 binary by Reid et al. 2006 (cat. J/AJ/132/891). The system's position in the midst of the main sequence in the HR diagram (Figure 4) shows that the system is dominated by the A component in luminosity. Our parallax observations detect a clear astrometric perturbation. We are working on mapping the system's orbit and will soon be able to publish dynamical masses for the individual components. Like LHS 2397aAB, this system will serve as a crucial benchmark system with components likely residing on either side of the stellar/substellar boundary. As indicated in Figure 9, this target has one of the largest optical variabilities in the sample, at 41mmag in I. We defer a more thorough discussion of 2MASS J1705-0516AB to a future paper (S. B. Dieterich et al., in preparation).
SIPS J2045-6332    M9.0V        ID 58 Is an extremely over-luminous object (Figure 4). We note that unresolved equal luminosity duplicity alone cannot explain the over-luminosity. The object is also highly variable at 39mmag in I, as shown in Figure 9. The variability suggests that youth may play a role in explaining the over-luminosity of SIPS J2045-6332.
LHS 4039C          M9.0V        ID 62 Is a member of a triple system with an M4V primary 102.8'' away from LHS 4039C. The third component is a DA white dwarf 6.5'' away from the primary (Scholz et al., 2004MNRAS.347..685S; Subasavage et al., 2009AJ....137.4547S). Subasavage et al. (2009AJ....137.4547S) report a trigonometric parallax of 43.74+/-1.43mas from the weighted mean of the A and B components. In this paper we have reduced the same data using LHS 4039C as the science target and measure a parallax of 44.38+/-2.09, thus supporting the physical association of the system. The intriguing combination of a white dwarf and a VLM star in the same system allows us to constrain the properties of LHS 4039C based on the better understood models of white dwarf evolution. Based on the white dwarf cooling time of 0.81+/-0.05Gyr (Subasavage et al., 2009AJ....137.4547S) and the progenitor age of 4.4+/-3.7Gyr (Iben & Laughlin, 1989ApJ...341..312I) assuming a progenitor mass of 1.17+/-0.26M_{sun}_ (Williams et al., 2009ApJ...693..355W), we infer a total system age of 5.2+/-3.7Gyr. Assuming the system to be coeval, LHS4039C is then a main sequence star with no remaining traces of youth. Its locus on the HR diagram is therefore an indication of where the VLM stellar main sequence lies (It was not possible to label this object in Figure 4 due to crowding of the diagram. The reader is referred to the online supplements where diagrams are plotted using ID numbers).