SN 2009au (SN II) spectrum around the Na I D lines. The black line is the spectrum, the blue lines are the estimated flux errors, the red stars are the nodes, and the dashed red line is the continuum crossing the nodes. Dotted red lines represent the continua at ±25% of the node separation being considered. The grey-shaded area is the integrated line within ±1000 km s⁻¹. The dashed black vertical line is the central wavelength of the line. The other vertical dotted blue lines indicate other wavelengths where we measured EW and whose dispersion provides a systematic uncertainty.