  S106 is one of the best known bipolar HII regions, thoroughly studied and
modelled at infrared, submillimeter and millimeter wavelengths, and it is one
of the nearest examples of the late stages of massive star formation in which
the newly formed star that ionizes it is still surrounded by vast amounts of
gas and dust. However, little is known about its heavily obscured central
source, S106IR. The possible binarity of the central source is investigated,
which is considered to be likely given the high binarity fraction among massive
stars. We have carried out visible and near-infrared photometric monitoring
looking for short-term variability, with special interest in that related to
the presence of a close binary companion to S106IR that may produce periodic
eclipses or tidal distortion of the shape of the members of the system. A
periodic variability of S106IR in the J band is found with a period of 5.0 days
and an amplitude of about 0.1 mag. The light curve displays a slow rise from
minimum to maximum followed by a steep decrease, and can be well reproduced by
a close binary system composed of two stars with different luminosity orbiting
each other in an elliptical orbit of moderate eccentricity. S106IR also shows
hints of short-term variability possibly related to accretion. We also report
variability of four other stars previously classified as members of the S106
cluster, all of which are strong X-ray emitters. The newly discovered close
binarity of S106IR adds a new element to the modeling of the nebula and to the
understanding of the dynamics of the gas around the ionizing source, which
suggests that the components of the binary are accreting via a circumbinary
disk. Binarity also helps to explain the apparent mismatch between the spectral
type of the ionizing source inferred from the nebular spectrum and its high
brightness at near-infrared wavelengths.
