A High Resolution Spectroscopic Observation of CAL 83 with   XMM-Newton/RGS
  We present the first high resolution photospheric X-ray spectrum of a Supersoft X-ray Source, the famous CAL~83 in the Large Magellanic Cloud. The spectrum was obtained with the Reflection Grating Spectrometer on XMM-Newton during the Calibration/Performance Verification phase of the observatory. The spectrum covers the range 20-40 A at an approximately constant resolution of 0.05 A, and shows very significant, intricate detail, that is very sensitive to the physical properties of the object. We present the results of an initial investigation of the spectrum, from which we draw the conclusion that the spectral structure is probably dominated by numerous absorption features due to transitions in the L-shells of the mid-$Z$ elements and the M-shell of Fe, in addition to a few strong K-shell features due to CNO. 
Frits Paerels, Andrew P. Rasmussen, H. W. Hartmann, J. Heise, A. C.   Brinkman, C. P. de Vries, and J.-W. den Herder