  ROSAT has discovered a new group of isolated neutron stars characterized by
soft black-body like spectra (kT ~ 50-120 eV), apparent absence of radio
emission and no association with supernovae remnants. So far only six such
sources are known. A small fraction of these stars exhibit X-ray pulsations
with relatively long periods of the order of 10 sec. Two very different
mechanisms may be envisaged to explain their properties. The neutron stars may
be old and re-heated by accretion from the ISM in which case their population
properties could provide information on past stellar formation and secular
magnetic field decay. Alternatively, this group may at least partly be made of
relatively young cooling neutron stars possibly descendant from magnetars. We
review the last observational results and show how they can shed light on the
evolutionary path of these new objects within the whole class of isolated
neutron stars.
