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Long term monitoring observations with the Hitachi 32-m radio telescope ofthe 6.7 GHz methanol masers associated with the high mass star-forming regionG5.900-0.430 is presented. A period of flux variability at approximately 1260days, is detected in the features at VLSR = 9.77 and 10.84 km/s while asecondary shorter period, 130.6 days, is determined for the 0.66 km/s feature.This is only the second source which has two different periods. The period of~1260 days is approximately twice as long as the longest known period of 6.7GHz methanol masers. The variability pattern of the symmetric sine curves andthe consistency with the expected period-luminosity relation suggest that themechanism of maser flux variability of 9.77 and 10.84 km/s features in thissource can be explained by protostellar pulsation instability. On the otherhand, because the 0.66 km/s feature has an intermittent and asymmetricvariability profile, we propose this feature is explained by the CWB or spiralshock models. Obtaining the spatial distribution of the 0.66 km/s feature usingan VLBI will lead to a better understanding of this source.

Detection of the longest periodic variability in 6.7 GHz methanol masers G5.900-0.430