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We report the discovery of an eclipsing companion to NLTT 41135, a nearby M5 dwarf that was already known to have a wider, slightly more massive common proper motion companion, NLTT 41136, at 2.''4 separation. Analysis of combined-light and RV curves of the system indicates that NLTT 41135B is a (31-34) +- 3M Jup brown dwarf (where the range depends on the unknown metallicity of the host star) on a circular orbit. The visual M dwarf pair appears to be physically bound, so the system forms a hierarchical triple, with masses approximately in the ratio 8:6:1. The eclipses are grazing, preventing an unambiguous measurement of the secondary radius, but follow-up observations of the secondary eclipse (e.g., with the James Webb Space Telescope) could permit measurements of the surface brightness ratio between the two objects, and thus place constraints on models of brown dwarfs.

We report the detection of eclipses in GJ 3236, a bright (I = 11.6), very low mass binary system with an orbital period of 0.77 days. Analysis of light and radial velocity curves of the system yielded component masses of 0.38 +- 0.02 M and 0.28 +- 0.02 M . The central values for the stellar radii are larger than the theoretical models predict for these masses, in agreement with the results for existing eclipsing binaries, although the present 5% observational uncertainties limit the significance of the larger radii to approximately 1s. Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii, and could be reduced by obtaining precise, multiband photometry covering the full orbital period. The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period, evidenced by strong Ha emission lines in the spectra of both components. These observations probe an important region of mass-radius parameter space around the predicted transition to fully convective stellar interiors, where there are a limited number of precise measurements available in the literature.

We present optical photometry of 16 transits of the super-Earth GJ 1214b, allowing us to refine the system parameters and search for additional planets via transit timing. Starspot-crossing events are detected in two light curves, and the star is found to be variable by a few percent. Hence, in our analysis, special attention is given to systematic errors that result from starspots. The planet-to-star radius ratio is 0.11610 +- 0.00048, subject to a possible upward bias by a few percent due to the unknown spot coverage. Even assuming this bias to be negligible, the mean density of the planet can be either 3.03 +- 0.50 g cm-3 or 1.89 +- 0.33 g cm-3, depending on whether the stellar radius is estimated from evolutionary models, or from an empirical mass-luminosity relation combined with the light curve parameters. One possible resolution is that the orbit is eccentric (e [?] 0.14), which would favor the higher density, and hence a much thinner atmosphere for the planet. The transit times were found to be periodic within about 15 s, ruling out the existence of any other super-Earths with periods within a factor of two of the known planet.

We present rotation period measurements for 41 field M-dwarfs, all of which have masses inferred (from their parallaxes and Two Micron All Sky Survey K-band magnitudes) to be between the hydrogen burning limit and 0.35 M and thus should remain fully convective throughout their lifetimes. We measure a wide range of rotation periods, from 0.28 to 154 days, with the latter commensurate with the typical sensitivity limit of our observations. Using kinematics as a proxy for age, we find that the majority of objects likely to be thick disk or halo members (and hence, on average, older) rotate very slowly, with a median period of 92 days, compared with 0.7 days for those likely to be thin disk members (on average, younger), although there are still some rapid rotators in the thick disk sample. When combined with literature measurements for M-dwarfs, these results indicate an increase in spin-down times with decreasing stellar mass, in agreement with previous work, and that the spin-down time becomes comparable to the age of the thick disk sample below the fully convective boundary. We additionally infer that the spin-down must remove a substantial amount of angular momentum once it begins in order to produce the slow rotators we observe in the thick disk candidates, suggesting that fully convective M-dwarfs may still experience strong winds.