Pulsating stars in $\omega$ Centauri: Near-IR properties and
  period-luminosity relations
  $\omega$ Centauri (NGC~5139) contains many variable stars of different types,
including the pulsating type II Cepheids, RR Lyrae and SX Phoenicis stars. We
carried out a deep, wide-field, near-infrared (IR) variability survey of
$\omega$ Cen, using the VISTA telescope. We assembled an unprecedented
homogeneous and complete $J$ and $K_{\rm S}$ near-IR catalog of variable stars
in the field of $\omega$ Cen. In this paper we compare optical and near-IR
light curves of RR Lyrae stars, emphasizing the main differences. Moreover, we
discuss the ability of near-IR observations to detect SX Phoenicis stars given
the fact that the amplitudes are much smaller in these bands compared to the
optical. Finally, we consider the case in which all the pulsating stars in the
three different variability types follow a single period-luminosity relation in
the near-IR bands.
