Ten new class II methanol masers from the 6.7-GHz Methanol Multibeam survey catalogues III and IV were selected for a monitoring programme at both 6.7 and 12.2 GHz with the 26m Hartebeesthoek Radio Astronomy Observatory ( HartRAO ) radio telescope for two years and nine months , from August 2012 to May 2015 .
In the sample , only masers associated with G339.986-0.425 were found to show periodic variability at both 6.7 and 12.2 GHz .
The existence of periodic variation was tested with four independent methods .
The analytical method gave the best estimation of the period , which was 246 \pm 1 days .
The time series of G339.986-0.425 show strong correlations across velocity channels and between the 6.7 and 12.2 GHz masers .
The time delay was also measured across channels and shows structure across the spectrum which is continuous between different maser components .