  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.
