Phase-resolved spectroscopy of the newly discovered X-ray transient MAXI J0556 - 332 has revealed the presence of narrow emission lines in the Bowen region that most likely arise on the surface of the mass donor star in this low mass X-ray binary .
A period search of the radial velocities of these lines provides two candidate orbital periods ( 16.43 \pm 0.12 and 9.754 \pm 0.048 hrs ) , which differ from any potential X-ray periods reported .
Assuming that MAXI J0556 - 332 is a relatively high inclination system that harbors a precessing accretion disk in order to explain its X-ray properties , it is only possible to obtain a consistent set of system parameters for the longer period .
These assumptions imply a mass ratio of q \simeq 0.45 , a radial velocity semi-amplitude of the secondary of K _ { 2 } \simeq 190 km s ^ { -1 } and a compact object mass of the order of the canonical neutron star mass , making a black hole nature for MAXI J0556 - 332 unlikely .
We also report the presence of strong N III emission lines in the spectrum , thereby inferring a high N/O abundance .
Finally we note that the strength of all emission lines shows a continuing decay over the \simeq 1 month of our observations .