SX Phoenicis stars are usually found in the outer regions of the galaxy which is called galactic halo. Their luminosity varies with a period of 1-2 hours. They are interesting because they are very old but have not become white dwarfs as expected. Fewer than 20 field SX Phoenicis stars are known to exist. By studying these stars, we can learn about how they formed, the makeup of the galactic halo, and possibly locate dwarf galaxies. The origin of SX Phe stars is still unknown.
Shuda, Suzanne, "SX Phoenicis Stars" (2015). Aerospace, Physics, and Space Science Student Publications. 15.