  We present the detection of RR Lyrae variable stars in the field of the
Sagittarius II (Sgr II) ultra-faint dwarf (UFD) galaxy. Using B, V time-series
photometry obtained with the Korea Microlensing Telescope Network (KMTNet) 1.6
m telescope at CTIO and G-band data from Gaia Data Release 2 (DR2), we
identified and characterized two ab-type and four c-type RR Lyrae variables.
Five out of the six stars are clustered within three half-light radii (~4.8')
of the galaxy indicating their association with Sgr II, while the RRab star V4
is located ~22' from the galaxy center. By excluding V4, the high c-type
fraction (0.8) and the period of the only RRab star V3 (P_V3 = 0.666 days)
suggest an Oosterhoff II (Oo II) classification for Sgr II. Located close to
the locus of Oo II clusters in the period-amplitude diagram, V3 is similar to
RRab stars in other UFDs having Oosterhoff-intermediate and Oo II properties.
Sgr II is, however, more compact than usual UFDs, placed in between star
clusters and dwarf galaxies in the size-luminosity plane, and therefore
spectroscopic studies are eventually required to ascertain the true nature of
this stellar system. We derive the metallicity ([Fe/H]_RRab = -2.1 +- 0.3) and
heliocentric distance (~64 +- 3 kpc) of Sgr II from the RR Lyrae stars, and
estimate its age (~12 Gyr) based on our stellar population models. The
Oosterhoff properties of UFDs can be explained with the evolution effect of RR
Lyrae stars in the instability strip.
