Diagnostic lines of 9 Sge that show the spectral variability of the star. Left panel: He I λ5876 line of 9 Sge at different epochs. It shows a P-Cygni profile that varies due to the effect of the stellar wind. Right panel: O III λ5592 line. It shows a more symmetric profile, indicating that the line is less affected by winds. Plotted spectra are: 040530_F (blue), 040827_P (green), 051110_P (orange), 110615_M_3 (red), and 110910_I_2 (purple). See Table A.1 for a description of naming convention used to identify each spectrum.