  We report on I band photometric observations of 21 stars with spectral types
between M8 and L4 made using the Isaac Newton Telescope. The total amount of
time for observations which had a cadence of <2.3 mins was 58.5 hrs, with
additional data with lower cadence. We test for photometric variability using
the Kruskal-Wallis H-test and find that 4 sources (2MASS J10224821+5825453,
2MASS J07464256+2000321, 2MASS J16262034+3925190 and 2MASS J12464678+4027150)
were found to be significantly variable at least on one epoch. Three of these
sources are reported as photometrically variable for the first time. If we
include sources which were deemed marginally variable, the number of variable
sources is 6 (29 percent). No flares were detected from any source. The
percentage of sources which we found were variable is similar to previous
studies. We summarise the mechanisms which have been put forward to explain the
light curves of brown dwarfs.
