  RX J0822-4300 is the Central Compact Object associated with the Puppis A
supernova remnant. Previous X-ray observations suggested RX J0822-4300 to be a
young neutron star with a weak dipole field and a peculiar surface temperature
distribution dominated by two antipodal spots with different temperatures and
sizes. An emission line at 0.8 keV was also detected. We performed a very deep
(130 ks) observation with XMM-Newton, which allowed us to study in detail the
phase-resolved properties of RX J0822-4300. Our new data confirm the existence
of a narrow spectral feature, best modelled as an emission line, only seen in
the `Soft' phase interval - when the cooler region is best aligned to the line
of sight. Surprisingly, comparison of our recent observations to the older ones
yields evidence for a variation in the emission line component, which can be
modelled as a decrease in the central energy from ~0.80 keV in 2001 to ~0.73
keV in 2009--2010. The line could be generated via cyclotron scattering of
thermal photons in an optically thin layer of gas, or - alternatively - it
could originate in low-rate accretion by a debris disk. In any case, a
variation in energy, pointing to a variation of the magnetic field in the line
emitting region, cannot be easily accounted for.
