  In this study, we examine the properties of donor stars of the three recently
discovered ultraluminous X-ray sources (ULXs) powered by rotating neutron
stars. For this purpose, we constructed a theoretical relation between the
X-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 of
donor stars in ULXs associated with neutron stars. Especially, comparing the
observed properties with the stellar evolution tracks, we suggest that the
donor 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 of
our models for the other two ULX systems (M82 X-2 and NGC7793 P-13) are
consistent with those in previous studies. Although there are only a few
samples, observed ULX systems with neutron stars seems to involve relatively
massive donors.
