  We analyse the K2 short cadence data of the intermediate polar FO Aqr and
provide accurate and updated orbital and spin periodicities. We additionally
find small spin period changes as a function of orbital phase of ~0.02 seconds
translating to velocities of ~ a few km/s. The obtained orbital-folded velocity
profile displays two clear maxima and minima, and cannot be explained by the
radial velocity of the orbiting white dwarf. Instead we propose that the
observed velocities are the sum of the radial velocities of both the white
dwarf and of the stellar surface facing the white dwarf which reprocesses the
WD spin pulses. This combination can explain the observed low velocities in FO
Aqr. However asymmetries in the orbital configuration are required to explain
the double peaked velocity profile. One possible scenario would invoke binary
eccentricity. We thus developed a simple binary model to explain and fit our
observations, and find a small binary eccentricity of e=0.03. Although small,
persistent eccentricity in a close interacting binary would induce enhanced
mass transfer occurring preferentially at periastron passages. We thus discuss
alternative scenarios where other asymmetries might explain our observations
assuming circular orbits. Since FO Aqr is the first system where the combined
radial velocities of both the WD and secondary surface have been measured, it
is possible that other mass-transferring binaries also display similar velocity
curves when observed with Kepler. These will provide additional valuable tests
to either confirm or rule out small eccentricities in similar systems.
