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 h. 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 latetype absorption features which match those of a G2V star. We find a semi-amplitude of 286±3 kms^-1 and a best-fitting orbital period of 0.198 0966(1) d. We combine these measurements with optical photometry which suggests the secondary star may be underfilling its Roche lobe by between 15 and 20 per cent. We weakly constrain the mass of the neutron star to be =2.2 M_⊙ at the 2σ level. We also discuss the possible origins of the H_α 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.