  GSN 069 is an ultra-soft X-ray active galactic nucleus that previously
exhibited a huge X-ray outburst and a subsequent long-term decay. It has
recently presented X-ray quasi-periodic eruptions (QPEs). We report the
detection of strong nitrogen lines but weak or undetectable carbon lines in its
far ultraviolet spectrum. With a detailed photoionization model, we use the C
IV/N IV] ratio and other ratios between nitrogen lines to constrain the [C/N]
abundance of GSN 069 to be from -3.33 to -1.91. We argue that a partially
disrupted red giant star can naturally explain the abnormal C/N abundance in
the UV spectrum, while the surviving core orbiting the black hole might produce
the QPEs.
