The Cygnus X region. XIII. The dark cloud between IC 1318b and c.

New observations are presented of the dark cloud between the emission nebulae IC 1318b and c in Cygnus. The H2CO- and H I-absorption lines and the H 110-alpha emission line, together with data from the literature are used to elaborate on the nature of the dark cloud. It is concluded that this dark cloud is the left-over of a much larger one, where star formation has ceased and where the dark cloud is being dissolved by an OB association, which is outlined by H II regions in the same line of sight. The age of the dark cloud must therefore be quite high (more than 100-million yr). Assuming that chemical equilibrium has been reached, the cloud would probably offer interesting possibilities for the observation of complex molecules. It is also suggested that the dark cloud may be the gamma-ray source CG0 78+01.