  We study the outburst of the newly discovered X-ray transient 3XMM
J185246.6+003317, re-analysing all available XMM-Newton, observations of the
source to perform a phase-coherent timing analysis, and derive updated values
of the period and period derivative. We find the source rotating at
P=11.55871346(6) s (90% confidence level; at epoch MJD 54728.7) but no evidence
for a period derivative in the 7 months of outburst decay spanned by the
observations. This translates in a 3sigma upper limit for the period derivative
of Pdot<1.4x10^{-13} s/s, which, assuming the classical magneto-dipolar braking
model, gives a limit on the dipolar magnetic field of B_dip<4.1x10^{13} G . The
X-ray outburst and spectral characteristics of 3XMM J185246.6+003317 confirms
the identification as a magnetar, but the magnetic field upper limit we derive
defines it as the third "low-B" magnetar discovered in the past three years,
after SGR 0418+5729 and Swift J1822.3-1606. We have also obtained an upper
limit to the quiescent luminosity (< 4x10^{33} erg/s), in line with the
expectations for an old magnetar. The discovery of this new low field magnetar
reaffirms the prediction of about one outburst per year from the hidden
population of aged magnetars.
