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We present optical photometric and spectroscopic observations of the likelystellar counterpart to the unassociated \emph{Fermi}-Large Area Telescope (LAT)$\gamma$-ray source 2FGL J0846.0+2820, selected for study based on positionalcoincidences of optical variables with unassociated LAT sources. Using opticalspectroscopy from the SOAR telescope, we have identified a late-G giant in aneccentric ($e$ = 0.06) 8.133 day orbit with an invisible primary. Modeling thespectroscopy and photometry together lead us to infer a heavy neutron starprimary of $\sim 2 M_{\odot}$ and a partially stripped giant secondary of $\sim0.8 M_{\odot}$. H$\alpha$ emission is observed in some of the spectra, perhapsconsistent with the presence of a faint accretion disk. We find the$\gamma$-ray flux of 2FGL J0846.0+2820 dropped substantially in mid-2009,accompanied by an increased variation in the optical brightness, and since thenit has not been detected by \emph{Fermi}. The long period and giant secondaryare reminiscent of the $\gamma$-ray bright binary 1FGL J1417.7--4407, whichhosts a millisecond pulsar apparently in the final stages of the pulsarrecycling process. The discovery of 2FGL J0846.0+2820 suggests theidentification of a new subclass of millisecond pulsar binaries that are thelikely progenitors of typical field millisecond pulsars.

2FGL J0846.0+2820: A new neutron star binary with a giant secondary and variable $\gamma$-ray emission