  Recently the first finding of a spin period of an accreting neutron star in
M31 is reported. The observed spin period is 1.2 s and it shows 1.27 d
modulations due to orbital motion. From the orbital information, the mass donor
could not be a giant massive star. On the other hand, the observed properties
are quite odd for typical low mass X-ray binaries. In this study, we compare
observed binary parameters with theoretical models given by a stellar evolution
track and make a restriction on the possible mass range of the donor. According
to the standard stellar evolution model, the donor star should be larger than
1.5 solar mass, and this suggests that this system is a new member of a rare
category, intermediate mass X-ray binary. The magnetic field strength of the
neutron star suggested by spin-up/down tendency in this system supports the
possibility of intermediate mass donor.
