2002A&A...388L..13S

Using the Ly_α-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 R_NC between the Ly_α-lines of nitrogen and carbon is below unity, while R_NC 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 R_NC-ratios as being due to the exposure of CNO-cycle processed material at the surfaces and in the coronae of Algol and β Cet.