We use surface brightness fluctuation ( SBF ) measurements to constrain the distance to low surface brightness ( LSB ) dwarfs in the vicinity of M101 .
Recent work has discovered many LSB candidate satellite companions of M101 .
However , without accurate distances , it is problematic to identify these dwarfs as physical satellites of M101 .
We use CFHT Legacy Survey ( CFHTLS ) data to measure the SBF signal for 43 candidate dwarfs .
The data is deep enough that we constrain 29 of these to be unassociated background galaxies by their lack of SBF .
We measure high S/N SBF signals for two of the candidate dwarfs , which are consistent with being at the distance of M101 .
The remaining candidates are too LSB and/or small for their distances to be constrained .
Still , by comparison with Local Group dwarfs , we argue that the M101 satellite system is likely now complete down to stellar masses of \sim 5 \times 10 ^ { 5 } M _ { \odot } .
We also provide a new SBF distance for the nearby dwarf UGC 8882 , which suggests that it is significantly outside of the virial radius of M101 and is thus not a physical satellite .
By constraining the distances to a majority of the candidates using only archival data , our work demonstrates the usefulness of SBF for nearby LSB galaxies and for studying the satellite systems of nearby massive galaxies .