tokens: We thank the anonymous referee for thoughtful comments and suggestions that improved this paper. We are grateful to D. Koester for sharing his grid of white dwarf atmosphere models. We thank J. J. Hermes for his insightful comments on the appropriate mass ranges for the WD fits. This paper is based on observations made with the NASA/ESA Hubble Space Telescope , obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #13354 and R.D.M. has been supported by NASA HST-GO-13354.001-A and NSF AST- 1714506. E.M.L. is funded by a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award No. AST-1801937. Facility: HST (COS). Software: MESA (Paxton et al. 2015 ), numpy (van der Walt et al. 2011 ), astropy (Astropy Collaboration et al. 2013 ; Price-Whelan et al. 2018 ), emcee (Foreman-Mackey et al. 2013 ), seaborn (Waskom et al. 2018 ), matplotlib (Hunter 2007 ), scipy (Jones et al. 2001 ), ipython (Pérez Granger 2007 ).
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