The observability of old isolated neutron stars.

The observability of old isolated neutron star accreting from the interstellar medium is discussed considering specifically the Position Sensive Proportional Counter on board of ROSAT. Is is fount that ∼ 5000 old neutron stars can be detected in the All Sky Survey, and ∼ 2 deg -2 in Deep Exposures. The key observationnal features of old accreting neutron stars are used to describe a selection procedure and to assess future observational evidence.