Carbon-oxygen WD merger remnants appear similar to PNe and can produce bright [O III] lines. In the left panel, we show evolutionary tracks (solid lines) of CO WD merger remnants from Schwab (2021) with a binary mass ratio of 0.9 and a range of total masses. For comparison, we overplot on the same panel evolutionary trajectories of typical post-main-sequence single stars (dotted lines) evolved with MESA from Farag et al. (2020). On the right panel, we show WD merger remnants’ L _{[O III]} from CLOUDY simulation as a function of cloud inner radii with a few different R _D/G and L _⋆. This is analogous to the PNe version of the plot (Figure 1); we overplot a low-mass star PN model with L _⋆,3 = 5L _⊙ and R _D/G = 1% here (dashed line) for comparison. The WD merger remnant nebulae should be brighter than PNe from low-mass stars in old stellar populations unless R _D/G ≳ 10%. The orange-shaded region reflects the observed PNLF bright-end cutoff at M ₅₀₀₇ = −4.53 ± 0.06 translated to line luminosity from Ciardullo (2012).