A sample of 54 6.7-GHz methanol masers was monitored using the Hartebeesthoek 26-m telescope during the period 1999 January -2003 March. The observations were taken at 1-2 week intervals, with daily observations when possible if a source was seen to be varying rapidly. It was found that the majority of the sources display a significant level of variability. The timerange of variations range from a few days up to several years. The types of behaviour observed included non-varying, monotonic increases or decreases, as well as aperiodic, quasi-periodic and periodic variations. Seven sources show clear evidence of periodicity, with periods ranging from 132 d up to 520 d.Seven sources, viz. G188.95+0.89, 196.45-1.68, G328.237-0.548, G331.13-0.24, G338.92-0.06, G339.62-0.12 and G9.62+0.20, show strong evidence of periodicity. Rigorous analysis of the periodicity is beyond the scope of this paper and will be presented in Goedhart, Gaylard & van der Walt (in preparation). The following sections describe the variability seen in the entire source sample. Variability indexEach maser spectrum consists of a number of peaks, corresponding to emission from maser spots at different velocities. Channels with no maser emission show the maxima and minima of the noise in the spectra. Velocity channels at the maximum of each peak were selected for the following analysis. Because the relation of the spatial structure of the maser to the velocity structure is not known in many cases, it was decided not to average the channels in each peak (to C 2004 RAS, MNRAS 355, 553-584Long-term monitoring of 6.7-GHz methanol masers 555

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Time series are presented for the Class II methanol maser source G12.89+0.49, which has been monitored for nine years at the Hartebeesthoek Radio Astronomy Observatory. The 12.2 and 6.7 GHz methanol masers were seen to exhibit rapid, correlated variations on time‐scales of less than a month. Daily monitoring has revealed that the variations have a periodic component with a period of 29.5 d. The period seems to be stable over the 110 cycles spanned by the time series. There are variations from cycle to cycle, with the peak of the flare occurring anywhere within an 11 d window, but the minima occur at the same phase of the cycle. Time delays of up to 5.7 d are seen between spectral features at 6.7 GHz and a delay of 1.1 d is seen between the dominant 12.2 GHz spectral feature and its 6.7 GHz counterpart.

The class II methanol maser source G9.62+0.20E has been monitored since 1999 at 6.7 GHz and since 2000 at 12.2 GHz. Six flares have been observed to date. These flares are periodic, with an interval of 246 d between flares. The duration of the flare is approximately 3 months, with maximum amplitude reached a month after the start of the flare.

We report the results of 10 years of monitoring of six regularly varying 6.7 GHz methanol masers using the Hartebeesthoek 26m telescope. Observations were done at intervals of 1-2 weeks, with faster sampling during flaring episodes. Four of the sources were also monitored at 12.2 GHz and show correlated variations. We find the Lomb-Scargle periodogram to be the most sensitive method to search for periodicity but possibly prone to false detections. Periods range from 132.8 days (with 26 cycles observed) to 509 days (with 7 cycles observed). Five of the sources show arguably periodic variations, while G331.13-0.24 shows strong periodicity in one peak, with large and variable delays in other peaks.