  We present the first Swift/XRT long-term monitoring of 2S 0114+650, a
wind-fed supergiant X-ray binary for which both orbital and superorbital
periods are known (P_orb~11.6d and P_sup~0.8d). Our campaign, summing up to ~
79ks, is the most intense and complete sampling of the X-ray light curve of
this source with a sensitive pointed X-ray instrument, and covers 17 orbital,
and 6 superorbital cycles. The combination of flexibility, sensitivity, and
soft X-ray coverage of XRT allowed us to confirm previously reported spectral
changes along the orbital cycle of the source and unveil the variability in its
spectral parameters as a function of the superorbital phase. For completeness,
we also report on a similar analysis carried out by exploiting XRT archival
data on three additional wind-fed supergiant X-ray binaries IGR J16418-4532,
IGR J16479-4514, and IGR J16493-4348. For these sources, the archival data
provided coverage along several superorbital cycles but our analysis could not
reveal any significant spectral variability.
