  We report on the first proper motion measurement in the supernova remnant RX
J1713.7-3946 using the XMM-Newton X-ray telescope on a 13 year time interval.
This expansion measurement is carried out in the south-east region of the
remnant where two sharp filament structures are observed. For the outermost
filament, the proper motion is $0.73\pm0.047_{\rm stat} \pm 0.069_{\rm syst}$
arcsec yr$^{-1}$ which is equivalent to a shock speed of 3500 km s$^{-1}$ at a
distance of 1 kpc. In contrast with the bright north-west region where the
shock is interacting with the border of the cavity, the shock in the south-east
region is probably expanding in the original ambient medium carved by the
progenitor and can be used to derive the current density at the shock and the
age of the remnant. In the case where the shock is evolving in a wind profile
($\rho \propto r^{-s}$, s=2) or in a uniform medium (s=0), we estimate an age
of 2300 yrs and 1800 yrs respectively for an ejecta power-law index n=9. The
specific case of n=7 and s=0, yields an age of 1550 yrs which would reconcile
RX J1713.7-3946 with the historical records of SN 393. In all scenarios, we
derive similar upstream densities of the order of 0.01 cm$^{-3}$, compatible
with the lack of thermal X-rays from the shocked ambient medium.
