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We report the discovery of a strong emission line near 24.8 angstrom (0.5keV) in the newly discovered X-ray binary system MAXI J0556-332 with thereflection grating spectrometer onboard the XMM-Newton observatory. The X-raylight curve morphology during these observations is complex and showsoccasional dipping behavior. Here we present time- and rate-selected spectrafrom the RGS and show that this strong emission line is unambiguously presentin all the XMM observations. The measured line center is consistent with theLy-alpha transition of N VII in the rest frame. While the spectra containimprints of absorption lines and edges, there appear to be no othersignificantly prominent narrow line due to the source itself, thus making theidentification of the 24.8 angstrom line uncertain. We discuss possiblephysical scenarios, including a gravitationally redshifted O VIII Ly-alpha lineoriginating at the surface of a neutron star or an unusual donor with anextremely high N/O abundance (>57) relative to solar, that may have producedthis comparatively strong emission line.

A strong emission line near 24.8 angstrom in the X-ray binary system MAXI J0556--332: gravitational redshift or unusual donor?