Mapping the outer bulge with RRab stars from the VVV Survey
  The VISTA Variables in the V\'ia L\'actea (VVV) is a near-IR time-domain
survey of the Galactic bulge and southern plane. One of the main goals of this
survey is to reveal the 3D structure of the Milky Way through their variable
stars. Particularly the RR Lyrae stars have been massively discovered in the
inner regions of the bulge ($-8^\circ \lesssim b \lesssim -1^\circ$) by optical
surveys such as OGLE and MACHO but leaving an unexplored window of more than
$\sim 47$ sq deg ($-10.0^\circ \lesssim \ell \lesssim +10.7^\circ$ and
$-10.3^\circ \lesssim b \lesssim -8.0^\circ$) observed by the VVV Survey. Our
goal is to characterize the RR Lyrae stars in the outer bulge in terms of their
periods, amplitudes, Fourier coefficients, and distances, in order to evaluate
the 3D structure of the bulge in this area. The distance distribution of RR
Lyrae stars will be compared to the one of red clump stars that is known to
trace a X-shaped structure in order to determine if these two different stellar
populations share the same Galactic distribution. We report the detection of
more than 1000 RR Lyrae ab-type stars in the VVV Survey located in the
outskirts of the Galactic bulge. Some of them are possibly associated with the
Sagittarius Dwarf Spheroidal Galaxy. We calculated colors, reddening,
extinction, and distances of the detected RR Lyrae stars in order to determine
the outer bulge 3D structure. Our main result is that, at the low galactic
latitudes mapped here, the RR Lyrae stars trace a centrally concentrated
spheroidal distribution. This is a noticeably different spatial distribution to
the one traced by red clump stars known to follow a bar and X-shape structure.
We estimate the completeness of our RRab sample in $80\%$ for $K_{\rm
s}\lesssim15$ mag.
