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We present spectroscopic and imaging observations of the active T dwarf 2MASS1237+6526, intended to investigate the emission mechanism of this cool browndwarf. The H$\alpha$ emission line first detected in 1999 July appears to bepersistent over 1.6 years, with no significant variation from$\log_{10}(L_{H{\alpha}}$/L$_{bol}$) = $-$4.3, ruling out flaring as a possiblesource. The relatively high level of emission in this object appears to beunique amongst observed late-L and T dwarfs. One of our spectra shows anapparent velocity shift in the H$\alpha$ line, which could be indicative of anaccretion hot spot in orbit around the brown dwarf; further confirmation ofthis shift is required. J-band monitoring observations fail to detect anysignificant variability (e.g., eclipsing events) at the $\pm$0.025 mag levelover periods of up to 2.5 hours, and there appears to be no statisticalevidence of variability for periods of up to 14 hours. These limits constrainthe mass of a hypothetical interacting secondary to M$_2$ $\lesssim$ 20M$_{Jup}$ for inclinations $i$ $\gtrsim$ 60$\degr$. While our observations donot explicitly rule out the binary hypothesis for this object, it does suggestthat other mechanisms, such as youthful accretion, may be responsible.Comment: 21 pages, 9 figures, accepted to AJ May 200

A Search for Variability in the Active T Dwarf 2MASS 1237+6526