  A comparison between the observed light curves of periodic masers in G9.62+0.20E and G188.95+0.89 and the results of a simple colliding-wind binary model is made to establish whether the flaring and other time-dependent behaviour of the masers in these two star forming regions can be ascribed to changes in the environment of the masers or in the continuum emission from parts of the background \ion{H}{2} region. It is found that the light curves of widely different shape and amplitude in these two objects can be explained within the framework of a periodic pulse of ionizing radiation that raises the electron density in a volume of partially ionized gas against which the masers are projected. It is also shown that the decay of the 11.405 $\kmps$ maser in G188.95+0.89 can be explained very well in terms of the recombination of the ionized gas against which the maser is projected while it would require very special conditions to explain it in terms of changes in environment of the maser. We conclude that for G9.62+0.20E and G188.95+0.89 the observed changes in the masers are most likely due to changes in the background free-free emission which is amplified by the masers. 
On the methanol masers in G9.62+0.20E and G188.95+0.89