  The possible existence of stellar halos in low-mass galaxies is being
intensely discussed nowadays after some recent discoveries of stars located in
the outskirts of dwarf galaxies of the Local Group. RR Lyrae stars can be used
to identify the extent of these structures, taking advantage of the
minimization of foreground contamination they provide. In this work we use RR
Lyrae stars obtained from Gaia DR3, DES, ZTF, and Pan-STARRS1 to explore the
outskirts of $45$ ultra-faint dwarf galaxies. We associate the stars with a
host galaxy based on their angular separations, magnitudes and proper motions.
We find a total of $120$ RR Lyrae stars that belong to $21$ different galaxies
in our sample. We report seven new RR Lyrae stars in six ultra-faint dwarf
galaxies (Hydrus I, Ursa Major I, Ursa Major II, Grus II, Eridanus II and
Tucana II). We found a large number of new possible members in Bootes I and
Bootes III as well, but some of them may actually belong to the nearby
Sagittarius stream. Adding to our list of $120$ RR Lyrae stars the observations
of other ultra-faint dwarf galaxies that were out of the reach of our search,
we find that at least $10$ of these galaxies have RR Lyrae stars located at
farther distances than $4$ times their respective half-light radius, which
implies that at least $33\%$ of the 30 ultra-faint dwarfs with RR Lyrae star
population have extended stellar populations.
