  We report on the results of high dispersion spectroscopy of two `superflare
stars', KIC 9766237, and KIC 9944137 with Subaru/HDS. Superflare stars are
G-type main sequence stars, but show gigantic flares compared to the Sun, which
have been recently discovered in the data obtained with the Kepler spacecraft.
Though most of these stars are thought to have a rotation period shorter than
10 days on the basis of photometric variabilities, the two targets of the
present paper are estimated to have a rotation period of 21.8 d, and 25.3 d.
Our spectroscopic results clarified that these stars have stellar parameters
similar to those of the Sun in terms of the effective temperature, surface
gravity, and metallicity. The projected rotational velocities derived by us are
consistent with the photometric rotation period, indicating a fairy high
inclination angle. The average strength of the magnetic field on the surface of
these stars are estimated to be 1-20 G, by using the absorption line of Ca II
8542. We could not detect any hint of binarity in our spectra, although more
data are needed to firmly rule out the presence of an unseen low-mass
companion. These results claim that the spectroscopic properties of these
superflare stars are very close to those of the Sun, and support the hypothesis
that the Sun might cause a superflare.
