Helium and carbon abundances in late-B and early-A supergiants

The abundances of carbon and helium were determined for representative late -B and early-A supergiants based on the CI lines (9078, 9089, 9095, 9112) i n the near-IR, CII lines (4267, 6151), and the HeI 6678 line, in an aim to investigate the nature of the envelope-mixing in these evolved stars based on the anomaly (if any) of these elements combined with that of N and O pub lished before. It turned out that N tends to increase with a decrease in C, showing a tendency of conserving the sum of CCN nuclei; which suggests tha t the anomaly of C and N may be reasonably interpreted as being due to mixi ng of the CN-processed material. However, this increase/decrease in N/C, in dicative of dredge-up of the H-burning product, is not accompanied by any H e-enrichment. Even surprisingly, the observed tendency is just the opposite , i.e., [He/H] appears to decrease progressively in accordance with a lower ing of [C/H]. Instead of regarding this apparent characteristics as being r eal, we tentatively speculate that some activity-related line-weakening mec hanism (e.g., irradiance of X-rays) might act on the formation of HeI lines , the extent of which is indirectly related to the efficiency of envelope m ixing via stellar rotation.