  HD 49798 (a hydrogen depleted subdwarf O6 star) with its massive white dwarf
(WD) companion has been suggested to be a progenitor candidate of type Ia
supernovae (SNe Ia). However, it is still uncertain whether the companion of HD
49798 is a carbon-oxygen (CO) WD or an oxygen-neon (ONe) WD. A CO WD will
explode as an SN Ia when its mass grows approach to Chandrasekhar mass, while
the outcome of an accreting ONe WD is likely to be a neutron star. We followed
a series of Monte Carlo binary population synthesis approach to simulate the
formation of ONe WD + He star systems. We found that there is almost no orbital
period as large as HD 49798 with its WD companion in these ONe WD + He star
systems based on our simulations, which means that the companion of HD 49798
might not be an ONe WD. We suggest that the companion of HD 49798 is most
likely a CO WD, which can be expected to increase its mass to the Chandrasekhar
mass limit by accreting He-rich material from HD 49798. Thus, HD 49798 with its
companion may produce an SN Ia in its future evolution.
