Very sensitive 21cm H I measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope ( GBT ) at an angular resolution of 9 \farcm 1 , with a 5 \sigma detection level of { N _ { \mathrm { H I } } } = 3.9 \times 10 ^ { 17 } cm ^ { -2 } \ > for a 30 km s ^ { -1 } \ > line .
Most of the H I in a 12 square degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and H I mass of 10 ^ { 5 } M _ { \sun } .
Their velocities in the Local Group Standard of Rest lie between -100 and +40 km s ^ { -1 } \ > , comparable to the systemic velocities of M31 and M33 .
The clouds appear to be isolated kinematically and spatially from each other .
The total H I mass of all nine clouds is 1.4 \times 10 ^ { 6 } M _ { \odot } for an adopted distance of 800 kpc with perhaps another 0.2 \times 10 ^ { 6 } M _ { \odot } in smaller clouds or more diffuse emission .
The H I mass of each cloud is typically three orders of magnitude less than the dynamical ( virial ) mass needed to bind the cloud gravitationally .
Although they have the size and H I mass of dwarf galaxies , the clouds are unlikely to be part of the satellite system of the Local Group as they lack stars .
To the north of M31 , sensitive H I measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud population to projected distances of \sim 100 kpc .
An extension of the M31 high-velocity cloud population at a similar distance to the south-east , toward M33 , is not observed .