2016MNRAS.456.4335M

10 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 26-m Hartebeesthoek Radio Astronomy Observatory radio telescope for 2 yr and 9 months, from 2012 August to 2015 May. 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±1 d. 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.