Ultra-faint dwarf (UFD) galaxies are by far the most common type of galaxy among the satellites of the Milky Way. The discovery of a few tens of UFD satellites in the Milky Way in the last few years provides a new opportunity to study the role of these small systems in the formation of the halo of our Galaxy. About 50 satellite galaxies are known today around the Milky Way. Finding RR Lyrae stars in these satellites is important since it allows to obtain the distance to these objects in an independent way. They also allow to get an insight on the stellar populations of UFDs and the role that they may have had in the formation of the halo of the Milky Way. In this talk I will describe our effort to increase the census of RR Lyrae stars in UFD galaxies. This includes our own observations for several systems as well Gaia DR2 data. Most UFDs host very few RR Lyrae stars; some systems have also extra-tidal RR Lyrae stars, an indication that the UFD is likely disrupting. Contrary to what is seen in the Galactic Halo, the RR Lyrae stars do not clearly follow any of the Oosterhoff group loci on a period-amplitude diagram. This is probably an indication that their contribution to the formation of the Galactic halo is a minor one.
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