PSR J2129?0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified Fermi gamma-ray sources. The pulsar companion is optically bright (mean mR?=?16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. We present 10 years of archival and new photometry of the companion from the Lincoln Near-Earth Asteroid Research Survey, the Catalina Real-time Transient Survey, the Palomar Transient Factory, the Palomar 60 inch, and the Las Cumbres Observatory Global Telescope. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200 inch indicates that the pulsar is massive: 1.74?±?0.18 . The G-type pulsar companion has mass 0.44?±?0.04 , one of the heaviest known redback companions. It is currently 95?±?1% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr?1 secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129?0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar–white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.
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