  The unprecedented spatial resolution of the Chandra observatory opens the
possibility to detect with relatively high accuracy proper motions at X-ray
wavelengths. We have conducted an astrometric study of three of the
"Magnificent Seven", the thermally emitting radio quiet isolated neutron stars
(INSs) discovered by ROSAT. These three INSs (RX J0420.0-5022, RX J0806.4-4123
and RX J1308.6+2127/RBS 1223) either lack an optical counterpart or have one
too faint to be used for astrometric purposes. We obtained ACIS observations 3
to 5 years apart to constrain or measure the displacement of the sources on the
X-ray sky using as reference the background of extragalactic or remote galactic
X-ray sources. Upper limits of 138 mas/yr and 76 mas/yr on the proper motion of
RX J0420.0-5022 and RX J0806.4-4123, respectively, have already been presented
in Motch et al. (2007). Here we report the very significant measurement (~ 10
sigma) of the proper motion of the third INS of our program, RX
J1308.6+2127/RBS1223. Comparing observations obtained in 2002 and 2007 reveals
a displacement of 1.1 arcsec implying a yearly proper motion of 223 mas, the
second fastest measured for the ROSAT discovered INSs. The source is rapidly
moving away from the galactic plane at a speed which precludes any significant
accretion of matter from the interstellar medium. Its transverse velocity of ~
740 (d/700pc) km/s might be the largest of the "Magnificent Seven" and among
the fastest recorded for neutron stars. RX J1308.6+2127/RBS1223 is thus a young
high velocity cooling neutron star. The source may have its origin in the
closest part of the Scutum OB2 association about 0.8 Myr ago, an age consistent
with that expected from cooling curves, but significantly younger than inferred
from pulse timing measurements (1.5 Myr).
