Recent calculations of atomic data for Fe XV have been used to generate theoretical line ratios involving n = 3-4 transitions in the soft X-ray spectral region (~52-83 ?), for a wide range of electron temperatures and densities applicable to solar and stellar coronal plasmas. A comparison of these with solar flare observations from a rocket-borne spectrograph (X-Ray Spectrometer/Spectrograph Telescope [XSST]) reveals generally good agreement between theory and experiment. In particular, the 82.76 ? emission line in the XSST spectrum is identified, for the first time to our knowledge in an astrophysical source, as the 3s 3d 3D3-3s 4p 3P2 transition of Fe XV. Most of the Fe XV transitions that are blended have had the species responsible clearly identified, although there remain a few instances in which this has not been possible. The line ratio calculations are also compared with a co-added spectrum of Capella obtained with the Chandra satellite, which is probably the highest signal-to-noise ratio observation achieved for a stellar source in the ~25-175 ? soft X-ray region. Good agreement is found between theory and experiment, indicating that the Fe XV lines are reliably detected in Chandra spectra and hence may be employed as diagnostics to determine the temperature and/or density of the emitting plasma. However, the line blending in the Chandra data is such that individual emission lines are difficult to measure accurately, and fluxes may only be reliably determined via detailed profile fitting of the observations. The co-added Capella spectrum is made available to hopefully encourage further exploration of the soft X-ray region in astronomical sources.
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