Discovery of periodic and alternating flares of the methanol and water
  masers in G107.298+5.639
  Methanol and water vapour masers are signposts of early stages of high-mass
star formation but it is generally thought that due to different excitation
processes they probe distinct parts of stellar environments. Here we present
observations of the intermediate-mass young stellar object G107.298+5.639,
revealing for the first time that 34.4 d flares of the 6.7 GHz methanol maser
emission alternate with flares of individual features of the 22 GHz water
maser. High angular resolution data reveal that a few components of both maser
species showing periodic behaviour coincide in position and velocity and all
the periodic water maser components appear in the methanol maser region of size
of 360 au. The maser flares could be caused by variations in the infrared
radiation field induced by cyclic accretion instabilities in a circumstellar or
protobinary disc. The observations do not support either the stellar pulsations
or the seed photon flux variations as the underlying mechanisms of the
periodicity in the source.
