We report the discovery of a new dwarf spheroidal satellite of M31, Andromeda IX, based on resolved stellar photometry from the Sloan Digital Sky Survey (SDSS). Using both SDSS and public archival data we have estimated its distance and other physical properties, and compared these to the properties of a previously known dwarf spheroidal companion, Andromeda V, also observed by SDSS. Andromeda IX is the lowest surface brightness galaxy found to date (mu_{V} ~ 26.8 mag arcsec^-2), and at the distance we estimate from the position of the tip of Andromeda IX's red giant branch, (m - M)_{0} ~ 24.5 (805 kpc), Andromeda IX would also be the faintest galaxy known (M_{V} ~ -8.3).
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