The VMC survey - XXVI. Structure of the Small Magellanic Cloud from RR
  Lyrae stars
  We present results from the analysis of 2997 fundamental mode RR Lyrae
variables located in the Small Magellanic Cloud (SMC). For these objects
near-infrared time-series photometry from the VISTA survey of the Magellanic
Clouds system (VMC) and visual light curves from the OGLE IV survey are
available. In this study the multi-epoch $K_{\rm s}$-band VMC photometry was
used for the first time to derive intensity-averaged magnitudes of the SMC RR
Lyrae stars. We determined individual distances to the RR Lyrae stars from the
near-infrared period-absolute magnitude-metallicity ($PM_{K_{\rm s}}Z$)
relation, which has a number of advantages in comparison with the visual
absolute magnitude-metallicity ($M_{V}-{\rm [Fe/H]}$) relation, such as a
smaller dependence of the luminosity on interstellar extinction, evolutionary
effects and metallicity. The distances we have obtained were used to study the
three-dimensional structure of the SMC. The distribution of the SMC RR Lyrae
stars is found to be ellipsoidal. The actual line-of-sight depth of the SMC is
in the range from 1 to 10 kpc, with an average depth of 4.3 $\pm$ 1.0 kpc. We
found that RR Lyrae stars in the eastern part of the SMC are affected by
interactions of the Magellanic Clouds. However, we do not see a clear
bimodality in the distribution of RR Lyrae stars as observed for red clump (RC)
stars.
