Discovery of X-ray emission lines from the RS CVn binary σ CrB

X-ray observations of the RS CVn binary σ CrB carried out with the solid state spectrometer on board the Einstein Observatory are reported. X-ray emission lines due to Mg XI, Si XIII, S XV and a blend of L-transition lines due to Fe XVII and Fe XVIII are clearly detected in the spectrum of σ CrB. The observed X-ray spectrum is best described by a two-temperature coronal X-ray emission model, the dominant low temperature component originating in a plasma at a temperature of (5.9 +- 0.5) x 106 K and the weaker higher-temperature component arising from a plasma of >= 35 x 106 K. The inferred abundances of the elements like Mg, Si and Fe are consistent with solar values. Intensity variations over a period of a few hours were also recorded during the observations. Implications of these results are discussed in terms of the constant-pressure coronal loop model of Rosner, Tucker and Vaiana. (author)