  We report on a detailed spectral analysis of all the available XMM-Newton
data of RX J1856.5-3754, the brightest and most extensively observed nearby,
thermally emitting neutron star. Very small variations (~1-2%) in the
single-blackbody temperature are detected, but are probably due to an
instrumental effect, since they correlate with the position of the source on
the detector. Restricting the analysis to a homogeneous subset of observations,
with the source at the same detector position, we place strong limits on
possible spectral or flux variations from March 2005 to present-day. A slightly
higher temperature (kT~61.5 eV, compared to the average value kT~61 eV) was
instead measured in April 2002. If this difference is not of instrumental
origin, it implies a rate of variation of about 0.15 eV/yr between April 2002
and March 2005. The high-statistics spectrum from the selected observations is
well fit by the sum of two blackbody models, which extrapolate to an optical
flux level in agreement with the observed value.
