  An intensive monitoring program of 54 6.7-GHz methanol maser sources was
carried out at the Hartebeesthoek Radio Astronomy Observatory from January 1999
to April 2003. The monitoring program was subsequently continued on 19 sources
of interest. Analysis of the resulting time-series stretching over eight years
shows that six of the sources are periodic, with periods ranging from 133 days
to 504 days. The waveforms in individual sources range from sinusoidal
fluctuations to sharp flares and there can be other long term trends in the
time-series. The amplitudes of the variations can also change from cycle to
cycle. The time-series of the periodic masers will be presented, and possible
causes of the variability discussed.
