  I present the results of a program to spectrophotometrically monitor the L1.5
dwarf 2MASSW J1145572+231730 to identify the cause of photospheric variability
in ultra cool dwarfs. Plausible candidates are magnetically-induced star spots
and inhomogeneous photospheric dust clouds. Based on the atmospheric models and
synthetic spectra of Allard et al. (2001), the expected signatures of these
phenomena in the 0.5-2.5 um wavelength region are presented and discussed. Near
infrared spectra of 2M1145 were obtained along with a nearby reference star
observed simultaneously in the spectrograph slit. No convincing variability
over a 54 hour interval is found in any one of several colour indices designed
to be most sensitive to dust- and spot-related variability. Nonetheless, a
significant correlation between the variability of two colour indices is found.
This is slightly more consistent with the dust-related variability model than
the cool spot one considered. Based on the theoretically predicted signatures
and the median errors in the colour indices (0.03-0.05 magnitudes), upper
limits are placed on the coverage of possible spots and clouds. Assuming the L
dwarf to be best modelled by a dusty atmosphere at 1900K, coherent clear clouds
are limited to a coverage of 10-15% of the projected surface area and 200K
cooler spots to a 20% coverage. A larger coverage of many small features
varying incoherently cannot be ruled out with this method. A lower effective
temperature restricts coherent clear clouds to be much smaller; a higher
temperature allows both clouds and spots to be larger. These upper limits are
consistent with the two separate variability detections in the I-band reported
by Bailer-Jones & Mundt (2001).
