Long-term multi-frequency maser observations of the intermediate-mass
  young stellar object G107.298+5.639
  Periodic flares of maser emission are thought to be induced either by
variations of the seed photon flux in young binary systems or the pump rate
regulated by stellar and accretion luminosities. We seek to study the
variability of four maser transitions of three different species in
G107.298+5.639 to constrain the dominant mechanism of periodic flares. Light
curves of the 6.7 GHz methanol and 22.2 GHz water vapour maser were obtained
with the Torun 32m radio telescope over 39 and 34 cycles, respectively. The
target was also monitored at the 1.6 GHz hydroxyl transitions with the
Nan\c{c}ay radio telescope over 13 cycles. All these maser lines were imaged
using VLBI arrays. The study confirms alternating flares of the methanol and
water masers with a period of 34.4d and reveals the synchronised behaviour of
the methanol and hydroxyl masers in this source. The observed spatial
distribution of the methanol maser cloudlets and the measured time delays of
the flares of individual features imply a ring-like structure of radius 240au
and thickness 30au. Internal proper motions indicate that the velocity of
methanol cloudlets is dominated by a disc-wind component of about 5km/s. The
methanol emission detected during only one VLBI observation is located in a
region about 550au from a central star, which also exhibits OH maser flares.
The erratic appearance of methanol features can be related to a powering object
of relatively low luminosity which, during some variability cycles, can excite
molecules only in the nearest part of the disc. A careful analysis of the maser
and infrared light curves reveal a strong correlation between the 6.7 GHz line
and the infrared flux densities supporting a radiative pumping of the maser.
The synchronised behaviour of the hydroxyl and methanol transitions indicates a
common pumping mechanism for the periodic flares of G107.298+5.639.
