  In search for the counterpart to the Fermi-LAT source 3FGL J0838.8-2829, we
performed a multi-wavelength campaign, in the X-ray band with Swift and
XMM-Newton, performed infrared, optical (with OAGH, ESO-NTT and IAC80) and
radio (ATCA) observations, as well as analysed archival hard X-ray data taken
by INTEGRAL. We report on three X-ray sources consistent with the position of
the Fermi-LAT source. We confirm the identification of the brightest object, RX
J0838-2827, as a magnetic cataclysmic variable, that we recognize as an
asynchronous system (not associated with the Fermi-LAT source). RX J0838-2827,
is extremely variable in the X-ray and optical bands, and timing analysis
reveals the presence of several periodicities modulating its X-ray and optical
emission. The most evident modulations are interpreted as due to the binary
system orbital period of ~1.64h and the white dwarf spin period of ~1.47h. A
strong flux modulation at ~15h is observed at all energy bands, consistent with
the beat frequency between spin and orbital periods. Optical spectra show
prominent Hbeta, HeI and HeII emission lines Doppler-modulated at the orbital
period and at the beat period. Therefore, RX J0838-2827, accretes through a
diskless configuration and could be either a strongly asynchronous polar or a
rare example of a pre-polar system in its way to reach synchronism. Among the
other two X-ray sources, XMM J083850.4-282759 showed a variable X-ray emission,
with a powerful flare lasting ~600s, similar to what is observed in
transitional millisecond pulsars during the sub-luminous disc state: that would
possibly associate this source with the Fermi-LAT source.
