Spectral energy distribution of the central star of Kn 26 including the optical ALBIREO spectrum (histogram) and broad-band optical, near-IR, and WISE W1 3.4 μm and W2 4.6 μm photometric measurements (diamonds). The optical ALBIREO spectrum is shown in further detail in the inset. In both plots, the smooth solid line represents the best fit to the optical spectrum by a white dwarf with temperature 70 000 K extincted by A_V = 0.65 mag ≡ c(Hβ) = 0.30. As discussed in the text, the emission excess in the 2MASS K_s and WISE W1 and W2 bands with respect to these two fits is due to the contribution of nebular emission to these bands, and thus they should be regarded as upper limits of the stellar emission.