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Almost every known low-luminosity Milky Way dwarf spheroidal (dSph) satellitegalaxy contains at least one RR Lyrae star. Assuming that a fraction of distant(60 < d_{helio} < 100 kpc) Galactic halo RR Lyrae stars are members of yet tobe discovered low-luminosity dSph galaxies, we perform a {\em guided} searchfor these low-luminosity dSph galaxies. In order to detect the presence of dSphgalaxies, we combine stars selected from more than 123 sightlines centered onRR Lyrae stars identified by the Palomar Transient Factory. We find that thismethod is sensitive enough to detect the presence of Segue 1-like galaxies(M_V= -1.5^{+0.6}_{-0.8}, r_h=30 pc) even if only ~20 sightlines were occupiedby such dSph galaxies. Yet, when our method is applied to the SDSS DR10 imagingcatalog, no signal is detected. An application of our method to sightlinesoccupied by pairs of close (<200 pc) horizontal branch stars, also did notyield a detection. Thus, we place upper limits on the number of low-luminositydSph galaxies with half-light radii from 30 pc to 120 pc, and in the probedvolume of the halo. Stronger constraints on the luminosity function may beobtained by applying our method to sightlines centered on RR Lyrae starsselected from the Pan-STARRS1 survey, and eventually, from LSST. In theAppendix, we present spectroscopic observations of an RRab star in theBo\"{o}tes 3 dSph and a light curve of an RRab star near the Bo\"{o}tes 2 dSph.

Stacking the Invisibles: A Guided Search for Low-Luminosity Milky Way Satellites