Carbon and nitrogen abundances in the coronae of Algol B and other evolved stars: Evidence for CNO-cycle processed material

Using the Ly(alpha)-lines of carbon and nitrogen measured in Chandra Low Energy Transmission Grating Spectrometer (LETGS) spectra we study the coronal abundances of these elements in a sample of late-type stars including dwarf stars, the prototypical eclipsing binary Algol, the single giant beta Cet, and three RS CVn binaries. In the main sequence stars of our sample the flux ratio R-NC between the Ly(alpha)-lines of nitrogen and carbon is below unity, while RNC is found to be approximate to8.5 for beta Cet and >23.3 for Algol. These values are more than an order of magnitude larger than expected from a solar abundance plasma in collisional equilibrium regardless of the chosen temperature structure. We therefore interpret the anomalously large R-NC-ratios as being due to the exposure of CNO-cycle processed material at the surfaces and in the coronae of Algol and beta Cet.