  Current models that explain giant (type II) X-ray outbursts in Be/X-ray
binaries (BeXB), are based on the idea of highly distorted disks. They are
believed to occur when a misaligned and warped disk becomes eccentric, allowing
the neutron star to capture a large amount of material. The BeXB 4U 0115+63
underwent two major outbursts in 2015 and 2017. Our aim is to investigate
whether the structural changes in the disk expected during type II outbursts
can be detected through optical polarimetry. We present the first optical
polarimetric observations and new optical spectra of the BeXB 4U 0115+63
covering the period 2013-2017. We study in detail the shape of the H$\alpha$
line profile and the polarization parameters before, during, and after the
occurrence of a type II X-ray outburst. We find significant changes in
polarization degree and polarization angle and highly distorted line profiles
during the 2017 X-ray outburst. The degree of polarization decreased by $\sim$
1%, while the polarization angle, which is supposed to be related with the disk
orientation, first increased by $\sim 10^{\circ}$ in about two months and then
decreased by a similar amount and on a similar timescale once the X-ray
activity ceased.We interpret the polarimetric and spectroscopic variability as
evidence for the presence of a warped disk.
