2 PARTIALLY OVERLAPPING CLOUDS OF DIFFERENT OPTICAL-PROPERTIES IN THEPERSEUS OB2 ASSOCIATION

The paper considers the properties of H I clouds seen in front of the two ''arms'' of the giant CO complex observed in Perseus (Per OB2 asso ciation - see Fig. 1). The clouds observed towards both ''arms'' show striking differences in the pattern of interstellar absorptions: the s hapes of the extinction curves derived from vacuum-UV photometric and spectroscopic data, the strength ratio of the two major diffuse inters tellar bands: 5780 and 5797 and, probably in the strengths of molecula r features. The diffuse band ratio which varies across the Per OB2 ass ociation suggests, together with the varying widths of sodium lines, t hat two, partially overlapping clouds, differing strongly in the optic al properties, are situated towards the aggregate. Only towards one of the ''arms'' we observe the absorption features of simple, interstell ar molecules such as CN or CH. The results suggest that at least the p eripheral layers of giant molecular clouds, giving birth to OB associa tions, are not well-mixed, containing thus H I clouds of different pro venience. The hypothesis that giant clouds are formed by gathering dif fuse interstellar clouds gets support.