  The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in
October 2015. The source did not decay to its quiescent state but settled in a
meta-stable plateau state (a factor ~10 brighter than quiescence) in which its
luminosity slowly decayed. We used XMM-Newton to observe the system during this
phase and we found that its spectrum can be well described using a black-body
model with a small emitting radius. This suggests emission from hot spots on
the surface, which is confirmed by the detection of pulsations. In addition, we
obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our
XMM-Newton one. We found that the source luminosity was significantly higher
and, although the spectrum could be fitted with a black-body model the
temperature was higher and the emitting radius smaller. Several weeks later the
system started a sequence of type-I accretion outbursts. In between those
outbursts, the source was marginally detected with a luminosity consistent with
its quiescent level. We discuss our results in the context of the three
proposed scenarios (accretion down to the magnestospheric boundary, direct
accretion onto neutron star magnetic poles or cooling of the neutron star
crust) to explain the plateau phase.
