  In the last 10 years, since its last giant outburst in 2006, regular X-ray
outbursts (type I) were detected every periastron passage in the Be X-ray
binary EXO 2030+375. Recently, however, it was reported that the source started
to show a peculiar behavior: its X-ray flux decreased significantly and type I
outbursts were missed in several cases. At the same time, the spin frequency of
the neutron star, which had been increasing steadily since the end of the 2006
giant outburst, reached a plateau. Very recent observations indicate that the
source is now starting to spin down. These observed phenomena have a striking
similarity with those which took place 20 years ago, just before the source
displayed a sudden orbital phase shift of the outburst peak (1995). This
historical event occurred at the time exactly between the two giant outbursts
(1985 and 2006). These phenomena suggest the system to have an underlying
periodicity of 10.5 years between orbital phase shifts and/or giant outbursts.
The suggested periodicity may reflect some long-period dynamics in the
circumstellar disk of the Be star, due, e.g., to the Kozai-Lidov effect. A
model generating such a periodic change of the Be disk, namely Kozai-Lidov
oscillations in the Be disk, is discussed. If this behavior is really
periodical, another phase shift of the X-ray outburst peak is predicted to
occur around 2016 December.
