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

Using the Lyα-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 β Cet, and three RS CVn binaries. In the main sequence stars of our sample the flux ratio RNC between the Lyα-lines of nitrogen and carbon is below unity, while RNC is found to be ≈8.5 for β 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 RNC-ratios as being due to the exposure of processed material at the surfaces and in the coronae of Algol and β Cet.