Recent monitoring programs of ultra cool field M and L dwarfs (low mass stars
or brown dwarfs) have uncovered low amplitude photometric I-band variations
which may be associated with an inhomogeneous distribution of photospheric
condensates. Further evidence hints that this distribution may evolve on very
short timescales, specifically of order a rotation period or less. In an
attempt to study this behaviour in more detail, we have carried out a pilot
program to monitor three L dwarfs in the near infrared where these objects are
significantly brighter than at shorter wavelengths. We present a robust data
analysis method for improving the precision and reliability of infrared
photometry. No significant variability was detected in either the J or Km bands
in 2M1439 and SDSS1203 above a peak-to-peak amplitude of 0.04 mag (0.08 mag for
2M1112). The main limiting factor in achieving lower detection limits is
suspected to be second order extinction effects in the Earth's atmosphere, on
account of the very different colours of the target and reference stars.
Suggestions are given for overcoming such effects which should improve the
sensitivity and reliability of infrared variability searches.Comment: MNRAS, in press (9 pages