We report the discovery of SDSS~J022932.28+713002.7, a nascent extremely
low-mass (ELM) white dwarf (WD) orbiting a massive (>1Mββ at 2Ο
confidence) companion with a period of 36 hours. We use a combination of
spectroscopy, including data from the ongoing SDSS-V survey, and photometry to
measure the stellar parameters for the primary pre-ELM white dwarf. The
lightcurve of the primary WD exhibits ellipsoidal variation, which we combine
with radial velocity data and PHOEBE binary simulations to estimate the
mass of the invisible companion. We find that the primary WD has mass M1β =
0.18β0.02+0.02β Mββ and the unseen secondary has mass M2β =
1.19β0.14+0.21β Mββ. The mass of the companion suggests that it is
most likely a near-Chandrasekhar mass white dwarf or a neutron star. It is
likely that the system recently went through a Roche lobe overflow from the
visible primary onto the invisible secondary. The dynamical configuration of
the binary is consistent with the theoretical evolutionary tracks for such
objects, and the primary is currently in its contraction phase. The measured
orbital period puts this system on a stable evolutionary path which, within a
few Gyrs, will lead to a contracted ELM white dwarf orbiting a massive compact
companion.Comment: 21 Pages, 8 Figure