tend to be systematically cooler by about 500 K than those derived from empirical determinations of T$_{eff}$ using objects with known parallax. These conclusions are not necessarily inconsistent if different spectral features probe a range of optical depths in the atmosphere.

Finally, we note the presence of several weak lines of Ti I that arise from energy states near 1.4 eV, even lower than those of the strong potassium lines. A strong Ti multiplet at 0.97 has also been seen in the spectra of M dwarfs up to at least M9 (Cushing et al. 2005). Unfortunately, these weak lines are impossible to trace after M9.

Fine-scale structure; the role of FeH and 2o
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The astronomical $J$-band is bounded by 2o absorption bands from terrestrial water vapor. It is therefore no surprise that high-temperature 2o (hot steam) transitions intrinsic to M, L and T dwarfs encroach far into the $J$-band from both the short and long wavelength ends. These so-called infrared water bands are difficult to model because millions of transitions are needed (Partridge & Schwenke 1997). Typically, models over-estimate the depth of the infrared water bands. In addition to 2o, some of the stronger non-atomic transition features are known 