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In this study, we examine the properties of donor stars of the three recentlydiscovered ultraluminous X-ray sources (ULXs) powered by rotating neutronstars. For this purpose, we constructed a theoretical relation between theX-ray luminosity ($L_{\rm{X}}$) and the orbital period ($P_{\rm{orb}}$)suitable for ULXs with neutron stars. By using this new $L_{\rm{X}} -P_{\rm{orb}}$ relation, we attempt to determine the currently unknown nature ofdonor stars in ULXs associated with neutron stars. Especially, comparing theobserved properties with the stellar evolution tracks, we suggest that thedonor star in the NGC5907 ULX-1 system is a moderately massive star with $6 -12 \rm{M}_{\odot}$, just departing from the main sequence phase. The results ofour models for the other two ULX systems (M82 X-2 and NGC7793 P-13) areconsistent with those in previous studies. Although there are only a fewsamples, observed ULX systems with neutron stars seems to involve relativelymassive donors.

Nature of donors in ultra-luminous X-ray binaries powered by neutron stars