  The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure)
and A13 are stellar overdensities at estimated heliocentric distances of $d
\sim 11$ kpc and 15 kpc observed at low Galactic latitudes towards the
anticenter of our Galaxy. While these overdensities were initially thought to
be remnants of a tidally-disrupted satellite galaxy, an alternate scenario is
that they are composed of stars from the Milky Way (MW) disk kicked out to
their current location due to interactions between a satellite galaxy and the
disk. To test this scenario, we study the stellar populations of the Monoceros
Ring and A13 by measuring the number of RR Lyrae and M giant stars associated
with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae
stars in the two structures and measure radial velocities to compare with
previously measured velocities for M giant stars in the regions of the
Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars
($f_{RR:MG}$) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae
stars (116 in the Monoceros Ring and 37 in A13) and find that both structures
have very low $f_{RR:MG}$. The results support a scenario in which stars in A13
and Mon/GASS formed in the MW disk. We discuss a possible association between
Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar
velocity distributions and $f_{RR:MG}$.
