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tokens: We thank Josh Bloom, Brian Metzger, Alison Miller, Peter Nugent, and Eliot Quataert for discussions, and the referee for helpful comments. K.J.S. is supported by NASA through the Astrophysics Theory Program (NNX17AG28G). D.B. thanks the UK Science and Technology Facilities Council for supporting his PhD. We thank Encarni Romero Colmenero and Petri Vaisanen for the Director’s Discretionary Time award on SALT. S.W.J. is supported in part by NSF award AST-1615455. The UCSC group is supported in part by NASA grant NNG17PX03C, NSF grant AST-1518052, the Gordon Betty Moore Foundation, the Heising-Simons Foundation, and by fellowships from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation to R.J.F. M.F. is supported by a Royal Society-Science Foundation Ireland University Research Fellowship. K.M. is supported by STFC through an Ernest Rutherford fellowship. W.E.K. is supported by an ESO Fellowship and the Excellence Cluster Universe, Technische Universität München, Boltzmannstrasse 2, D-85748 Garching, Germany. Support for J.S. is provided by NASA through Hubble Fellowship grant # HST -HF2-51382.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. We acknowledge the “Observational Signatures of Type Ia Supernova Progenitors III” workshop at the Lorentz Center, where some of the preparatory work for this study was performed. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The NOT data were obtained through NUTS, which is supported in part by the Instrument Center for Danish Astrophysics (IDA). Support also comes from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 320964 (WDTracer). Some of the observations reported in this paper were obtained with the Southern African Large Telescope (SALT). PyRAF is a product of the Space Telescope Science Institute, which is operated by AURA for NASA. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University, and the Australian Astronomical Observatory. SkyMapper is owned and operated by The Australian National University’s Research School of Astronomy and Astrophysics. The survey data were processed and provided by the SkyMapper Team at ANU. The SkyMapper node of the All-Sky Virtual Observatory (ASVO) is hosted at the National Computational Infrastructure (NCI). Development and support the SkyMapper node of the ASVO has been funded in part by Astronomy Australia Limited (AAL) and the Australian Government through the Commonwealth’s Education Investment Fund (EIF) and National Collaborative Research Infrastructure Strategy (NCRIS), particularly the National eResearch Collaboration Tools and Resources (NeCTAR) and the Australian National Data Service Projects (ANDS). Software: rvsao (Kurtz Mink 1998 ), astropy (The Astropy Collaboration et al. 2018 ), galpy (Bovy 2015 ), MESA (v10108; Paxton et al. 2011 , 2013 , 2015 , 2018 ), PySALT ( http://pysalt.salt.ac.za ; Crawford et al. 2010 ), PyRAF ( http://www.stsci.edu/institute/software_hardware/pyraf ), ALFOSCGUI ( http://sngroup.oapd.inaf.it/foscgui.html ).