We present the unexpected discovery of four ultra diffuse galaxies ( UDGs ) in a group environment .
We recently identified seven extremely low surface brightness galaxies in the vicinity of the spiral galaxy M101 , using data from the Dragonfly Telephoto Array .
The galaxies have effective radii of 10 ^ { \prime \prime } -38 ^ { \prime \prime } and central surface brightnesses of 25.6 - 27.7 mag arcsec ^ { -2 } in g-band .
We subsequently obtained follow-up observations with HST to constrain the distances to these galaxies .
Four remain persistently unresolved even with the spatial resolution of HST /ACS , which implies distances of D > 17.5 Mpc .
We show that the galaxies are most likely associated with a background group at \sim 27 Mpc containing the massive ellipticals NGC 5485 and NGC 5473 .
At this distance , the galaxies have sizes of 2.6 - 4.9 kpc , and are classified as UDGs , similar to the populations that have been revealed in clusters such as Coma , Virgo and Fornax , yet even more diffuse .
The discovery of four UDGs in a galaxy group demonstrates that the UDG phenomenon is not exclusive to cluster environments .
Furthermore , their morphologies seem less regular than those of the cluster populations , which may suggest a different formation mechanism or be indicative of a threshold in surface density below which UDGs are unable to maintain stability .