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tokens: order of 100,000 white dwarfs with stellar mass companions, meaning that such systems are far from extraordinary. Furthermore, many WDMS systems will be post-common-envelope systems (perhaps still even being surrounded by the remnant common envelope in the form of a planetary nebula; Jones Boffin 2017 ; Boffin Jones 2019 ), which may lead to increased local extinction as a result of dust formation in the ejected envelope (Lü et al. 2013 ). As such, they are rather appropriate systems to constrain the importance of extinction in deriving their stellar parameters. An important caveat that must be noted is that many of these WDMS systems could also exhibit significant levels of irradiation—a problematic effect when it comes to the treatment of extinction as irradiation alters the emergent stellar spectrum, while here we only consider non-irradiated atmosphere models (see the discussion of Horvat et al. 2019 for more details). 10 10 As the discussion here is considered only illustrative, we fix the bolometric albedos and bolometric limb-darkening coefficients of both components to standard values. As a starting point, let us take one of the best constrained eclipsing post-common-envelope binaries known: SDSS J235524.29+044855.7 (hereinafter referred to as SDSS J2355; Rebassa-Mansergas et al. 2019 ). SDSS J2355 is a short-period post-common-envelope binary comprising a relatively cool white dwarf ( K) and a low-mass, metal-poor, subdwarf star (spectral type ∼sdK7). As before, calculating synthetic light curves for the system with no extinction and then with extinction consistent with the Galactic bulge, we now see significant deviations between the two models in u , g, and r bands (See Figure 4 ). The u band shows the strongest difference at more than 0.25 mag during primary eclipse, closely followed by the g band (at 0.13 mag) and the r band (at 0.02 mag, approximately 4 σ ). Even in the i band, the deviation is almost 0.01 mag—representing a more than 1 σ difference. The deviation during secondary eclipse would not be detected in any band given that the eclipse is not complete and the primary is far more luminous. Figure 4. Synthetic LSST light curves of the white-dwarf–subdwarf binary SDSS J2355 (Rebassa-Mansergas et al. 2019 ), calculated without (dashed line) and with extinction (solid line) corresponding approximately to the average extinction of the Galactic bulge. The residuals between extincted and unextincted cases are shown below each light curve, highlighting that the difference would be detectable at several σ by LSST in u , g , and r bands and marginally detected in the i band (in each residual plot, the shaded region represents ). 4. PHOENIX Atmospheres To increase the fidelity of computed observables on the low-temperature end, PHOEBE 2.2 now incorporates PHOENIX model atmospheres (Hauschildt et al. 1997 ) as computed by Husser et al. ( 2013 ). PHOENIX is one of the leading alternatives to Castelli Kurucz that incorporates 3D