Radial velocity of the peak emission of ¹²C¹⁶O lines as a function of the lower-level excitation energy of the transition. Filled symbols are from 2020 August, open symbols are from 2020 September. The dashed line is a linear fit to the data. The systemic velocity of the binary is −24.9 ± 0.2 km s⁻¹ (Gromadzki & Mikołajewska 2009) with the Mira velocity ∼−24 km s⁻¹ and the WD velocity ∼−27 km s⁻¹ on the dates of the observations.