Remnant molecular clouds in the ORI OB 1 association

We suggest and give some evidence that as an HII region expands and the O star(s) evolve(s) into B giant(s) the remains of the molecular clouds first appear as bright-rimmed clouds, then as cometary globules and finally as small clouds which are visible by the reflected light from the B giants. We propose to call the last one `reflection clouds' and all three categories collectively `remnant clouds'. A list is presented of about 80 objects of these remnant clouds in the Ori OB 1 association. In the Belt region there is a beautiful spatial sequence from bright-rimmed clouds through cometary globules to reflection clouds. We suspect that retarded star formation in remnant clouds can explain the presence of so-called dispersed T Tau stars in the peripheries of OB associations.