Gemini time-resolved spectroscopy of the Na I doublet and the H-alpha line over 90 minutes

Gemini time-resolved spectroscopy of the Na I doublet (top) and the H-alpha line (bottom) over 90 minutes. The Na I lines and the H-alpha line from the M dwarf are stationary, whereas the H-alpha line from the WD clearly shows a 20 minute periodicity.