All Figures

	Figure 1: Density-sensitive Fe  XXI line flux ratios as predicted by the APEC database for the temperature range between log(T)=6.6( lower borders) and 7.6 ( upper borders). In the right panel we show the term diagram explaining the formation of the 102.22 Å line as an example.
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In the text

Figure 2: Comparison of theoretical predictions for density sensitive f/i ratios for O  VII ( top panel) and Ne  IX ( bottom panel) from Eq. (2), the CHIANTI database, Porquet et al. (2001), and the APEC database. Filled areas represent the varying electron temperatures as given in the legends (hatched: CHIANTI, dark-grey shaded: Porquet et al. (2001), light-shaded: APEC). Lower temperatures yield lower f/i ratios.

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In the text

	Figure 3: Comparison of ROSAT luminosities (5.2-124 Å) listed in Table 2 with measurements for RGS1, RGS2, LETGS, MEG ( 5.15-38.2 Å), and HEG ( 5.15-21.5 Å) from Ness et al. (2003b) and Table 1. The regression curve (ROSAT) =  (Chandra/XMM) with parameters R1 and R2 as given in the legend is overplotted. The slope R2 is near unity, indicating good cross-calibration between instruments.
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In the text

	Figure 4: Line flux ratios of H-like (Ly) and He-like (r) lines vs. activity indicator for the respective ions of oxygen, neon, magnesium, and silicon. Linear fits yield good approximations with decreasing slopes for higher-Z ions. The right axes give average temperatures derived from comparison of the measured line ratios with temperature-sensitive predictions from the APEC line database.
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In the text

	Figure 5: Measurement of Fe  XXI lines for Algol and Capella. At most marginal evidence for the presence of Fe  XXI lines at 121.21 Å ( left panel) and at 142.16 Å as well as 145.65 Å ( right panel) is present. The 128.73 Å line is shifted due to calibration uncertainties. The grey shaded areas indicate the expected spectrum for high density plasma as found by Dupree et al. (1993).
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In the text

	Figure 6: Line flux ratios for measured Fe  XXI lines at 97.9 Å, 102.2 Å, 117.5 Å, 121.21 Å, and 128.8 Å. The grey lines represent the theoretical low-density limits. The average ratios, weighted with the measurement errors, are marked with the black lines.
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In the text

	Figure 7: Comparison of total X-ray luminosity and O  VII ( left) and Ne  IX-specific luminosities. The dashed lines indicate lines of equal luminosity.
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In the text

	Figure 8: Iron lines blending the Ne  IX intercombination line in high-temperature plasmas predicted by the APEC database assuming solar elemental abundances.
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In the text

	Figure 9: Comparison of coronal volumes obtained from O  VII and Ne  IX (only MEG measurements). The solid line marks the line of equal volumes.
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In the text

	Figure 10: Comparison of temperatures derived from Eq. (6) and from Si H-like and He-like line ratios representing the hot component for those stars where HETGS and LETGS spectra are available.
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In the text

Figure 11: Measured f/i ratios and derived densities versus the O  VII specific X-ray luminosities ( upper two panels). The low-density limit is marked with a vertical dotted line and measurements with only upper limits are marked by light symbols. Lower left panel: densities versus O  VII specific emission measure at T=2 MK. Lower right: emitting volume versus available volume (Eq. (7) with from Eq. (6); numbers are listed in Tables A.2/A.5, A.1/A.4, and A.3/A.6).

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In the text

	Figure 12: Same as Fig. 11 for Ne  IX.
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In the text

	Figure 13: Filling factors obtained from ratios of coronal volumes (derived from O  VII and Ne  IX densities) and assumed available volumes. The latter depend on assumed loop-top temperatures, which we derive from the H-like and He-like line ratios (marked with red bullets) and from (Eq. (6)). As activity indicator we use here the surface fluxes from .
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In the text