We report the discovery of DGSAT I , an ultra-diffuse , quenched galaxy located 10 \fdg 4 degrees in projection from the Andromeda galaxy ( M31 ) .
This low-surface brightness galaxy ( \mu _ { V } = 24.8 \mathrm { mag arcsec ^ { -2 } } ) , found with a small amateur telescope , appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images , and hence has been missed by optical surveys relying on resolved star counts , in spite of its relatively large effective radius ( R _ { e } ( V ) = 12 \arcsec ) and proximity ( 15′ ) to the well-known dwarf spheroidal galaxy And II .
Its red color ( V - I = 1.0 ) , shallow Sérsic index ( n _ { V } = 0.68 ) , and the absence of detectable \mathrm { H } \alpha emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars .

Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe , our radial velocity measurement obtained with the BTA 6-meter telescope ( V _ { h } = 5450 \pm 40 \mathrm { km s ^ { -1 } } ) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster .
At the distance of this cluster ( \sim 78 \mathrm { Mpc } ) , DGSAT I would have an R _ { e } \sim 4.7 \mathrm { kpc } and M _ { V } \sim - 16.3 .
Its properties resemble those of the ultra-diffuse galaxies recently discovered in the Coma cluster .
DGSAT I is the first case of these rare ultra-diffuse galaxies found in this galaxy cluster .
Unlike the ultra-diffuse galaxies associated with the Coma and Virgo clusters , DGSAT I is found in a much lower density environment , which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy .