3 Observations and data analysis

The LETGS onboard Chandra covers the spectral range between 6 Å-170 Å (i.e., including the spectral range covered with the ROSAT PSPC) with a spectral resolution of $\approx$0.03 Å; a detailed description of this instrument is given by Predehl et al. (1997). In Fig. 1 we show portions of the accumulated LETGS spectra covering the O  VII triplet and the nitrogen Ly$_{\alpha }$-line (left panel) and the carbon Ly$_{\alpha }$-line (right panel) for our sample stars Algol (=$\beta$ Per), the single giant $\beta$ Cet (spectral type K0III), the RS CVn binary UX Ari (spectral type G5V+K0V), and the nearby subgiant star Procyon (spectral type F5IV-V); the LETGS spectra of Procyon, Algol, and YY Gem have been discussed by Ness et al. (2001), Ness et al. (2002), and by Stelzer et al. (1998), respectively. The O  VII triplet and the nitrogen Ly$_{\alpha }$-line are seen in all stars, while the strength of the carbon Ly$_{\alpha }$-line varies considerably. In Procyon the strength of carbon Ly$_{\alpha }$ exceeds that of nitrogen Ly$_{\alpha }$, in Algol the carbon Ly$_{\alpha }$-line cannot be detected. In Table 1 we list the observed number of counts (and their errors) in the carbon and nitrogen Ly$_{\alpha }$-lines and their flux ratio  $R_{\rm NC}$; the line strengths were determined with the same methods as discussed by Ness et al. (2001). In addition to the stars shown in Fig. 1 we included Capella (spectral type G1III+G8/K0III) and HR 1099 (spectral type K1IV+G5IV), two RS CVn binaries, and the dwarf stars $\epsilon$ Eri (spectral type K2V), $\alpha$ Cen A (spectral type G2V) and B (spectral type K0V), YY Gem (spectral type dM1e+dM1e), $\pi^1$ UMa (spectral type G1.5V), and AD Leo (spectral type dM4.5Ve). In Fig. 2 we plot our measurements and a theoretical curve (solid line) of the expected ratio  $R_{\rm NC}$ of the nitrogen and carbon Ly$_{\alpha }$-line fluxes as a function of plasma temperature (solid line in Fig. 2 as calculated by Mewe et al. (1985) under the assumption of collisional equilibrium and cosmic abundances). Under those assumptions one expects $R_{\rm NC} \le 0.57$ independent of the actual plasma temperature (cf., Fig. 2). Since the precise plasma temperature is not known, and in fact a whole distribution of plasma at different temperatures is likely to be present in stellar coronae, the true value of  $R_{\rm NC}$ should be <0.57.

Figure 2: LETGS flux ratios of N  VII and C  VI for twelve stars compared to MEKAL flux ratio vs. temperature T (solid line). Algol, $\beta$ Cet, UX Ari, Capella, and HD 1099 lie significantly above the MEKAL curve for any temperature; the other stars are consistent with the MEKAL curve.