Chandra LETGS Observation of the Active Binary Algol
  A high-resolution spectrum obtained with the low-energy transmission grating onboard the Chandra observatory is presented and analyzed. Our analysis indicates very hot plasma with temperatures up to T=15-20MK from the continuum and from ratios of hydrogen-like and helium-like ions of Si, Mg, and Ne. In addition lower temperature material is present since OVII and NVI are detected. Two methods for density diagnostics are applied. The He-like triplets from NVII to SiXIII are used and densities around 10^11 cm^-3 are found for the low temperature ions. Taking the UV radiation field from the B star companion into account, we find that the low-Z ions can be affected by the radiation field quite strongly, such that densities of 3 10^10 cm^-3 are also possible, but only assuming that the emitting plasma is immersed in the radiation field. For the high temperature He-like ions only low density limits are found. Using ratios of FeXXI lines produced at similar temperatures are sensitive to lower densities but again yield only low density limits. We thus conclude that the hot plasma has densities below 10^12 cm^-3. Assuming a constant pressure corona we show that the characteristic loop sizes must be small compared to the stellar radius and that filling factors below 0.1 are unlikely. 
J.-U. Ness, J.H.M.M. Schmitt, V. Burwitz, R. Mewe, P. Predehl