PSR J1306–40 is a millisecond pulsar binary with a non-degenerate companion in an unusually long \sim 1.097 day orbit .
We present new optical photometry and spectroscopy of this system , and model these data to constrain fundamental properties of the binary such as the component masses and distance .
The optical data imply a minimum neutron star mass of 1.75 \pm 0.09 M _ { \odot } ( 1-sigma ) and a high , nearly edge-on inclination .
The light curves suggest a large hot spot on the companion , suggestive of a portion of the pulsar wind being channeled to the stellar surface by the magnetic field of the secondary , mediated via an intrabinary shock .
The H \alpha line profiles switch rapidly from emission to absorption near companion inferior conjunction , consistent with an eclipse of the compact emission region at these phases .
At our optically-inferred distance of 4.7 \pm 0.5 kpc , the X-ray luminosity is \sim 10 ^ { 33 } erg s ^ { \textrm { -1 } } , brighter than nearly all known redbacks in the pulsar state .
The long period , subgiant-like secondary , and luminous X-ray emission suggest this system may be part of the expanding class of millisecond pulsar binaries that are progenitors to typical field pulsar–white dwarf binaries .