SS Cyg is a dwarf nova binary star that undergoes frequent large amplitude outburst within a period of 40 days. SS Cyg was observed with the Chandra x-ray telescope in quiescence on 2000 August 24, and near the peak and early decline of a narrow outburst on 2000 September 12 and 14 (exposures of 47 and 96 ks). The spectrum of the source shows strong emission lines of O VII and O VIII in all three observations. These lines, we note, originate from the boundary layer.
EZE, ROMANUS N. C. (2007) "The Origin of X-Ray Emission Lines From SS Cgy," Turkish Journal of Physics: Vol. 31: No. 4, Article 7. Available at: https://journals.tubitak.gov.tr/physics/vol31/iss4/7