Roche Lobe Underfilling of the Secondary Star in PSR J102347.6+003841?

PSR J102347.6+003841 is a radio pulsar system with a spin period of 1.69 ms and an orbital period of 4.75 hours. Uniquely, it undergoes periods of transient accretion from its companion star: it occupies an important position in the evolutionary track from X-ray binary to isolated millisecond radio pulsar. Here we present a spectroscopic study of this system showing late-type absorption features which match those of a G2V star. We find a semiamplitude of $286\pm 3$ kms${}^{-1}$ and a best fit orbital period of 0.1980966(1) days. We combine these measurements with optical photometry which suggests the secondary star may be underfilling its Roche lobe by between 15% and 20%. We weakly constrain the mass of the neutron star to be $\le$ 2.2 M${}_{\odot }$ at the 2$\sigma$ level. We also discuss the possible origins of the H$\alpha$ emission line in our template subtracted, averaged spectrum. Finally we present and discuss new optical photometry of J1023 taken during the recent outburst of the system.