  The excellent spatial resolution of the Chandra observatory offers the
unprecedented possibility to measure proper motions at X-ray wavelength with
relatively high accuracy using as reference the background of extragalactic or
remote galactic X-ray sources. We took advantage of this capability to
constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray
bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and
lacking an optical counterpart. In this paper, we present results from a
preliminary analysis from which we derive 2 sigma upper limits of 76 mas/yr and
138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022
respectively. We use these values together with those of other ROSAT discovered
INSs to constrain the origin, distance and evolutionary status of this
particular group of objects. We find that the tangential velocities of radio
quiet ROSAT neutron stars are probably consistent with those of 'normal'
pulsars. Their distribution on the sky and, for those having accurate proper
motion vectors, their possible birth places, all point to a local population,
probably created in the part of the Gould Belt nearest to the earth.
