Logarithmic plots of total column densities along lines of sight in Cep OB2 and Cep OB3. The error bars represent 10% uncertainties in N. Top: N(CH) vs. N(H₂). The solid line shows the best fit to Cep OB2 data. Bottom: N(CO) vs. N(H₂). The solid line represents the best fit to Cep OB2 data, and the dashed line to Cep OB3 data. Note that, for a given N(H₂), Cep OB2 sight lines have higher CH and CO column densities than lines of sight toward Cep OB3.