We present H i imaging of GBT 1355+5439 performed with the Westerbork Synthesis Radio Telescope .
This is a dark H i object recently discovered close to the nearby galaxy M101 .
We find GBT 1355+5439 to be an H i cloud 5 \times 3 arcmin in size .
The total H i image and the kinematics show that the cloud consists of condensations that have small ( \sim 10 km s ^ { -1 } ) motions with respect to each other .
The column densities of the H i are low ; the observed peak value is 7.1 \times 10 ^ { 19 } cm ^ { -2 } .
The velocity field shows a mild velocity gradient over the body of GBT 1355+5439 , possibly due to rotation , but it may also indicate large-scale radial motions .
Although our data are limited in sensitivity , at all positions the H i velocity dispersion is higher than 5 km s ^ { -1 } and no narrow , cold , H i component is seen .
Because its distance is not known , we considered various possibilities for the nature of GBT 1355+5439 .
Both the scenarios that it is a tidal remnant near M101 and that it is a dark dwarf companion of M101 meet difficulties .
Neither do the data fit the properties of known compact High-Velocity Clouds in the Galactic halo exactly , but we can not entirely exclude this option and deeper observations are required .
We also considered the possibility that GBT 1355+5439 is a gas-rich dark minihalo in the outer regions of the Local Group .
Interestingly , it would then have similar properties as the clouds of a proposed Local Group population recently found in the ALFALFA survey .
In this case , the H i mass of GBT 1355+5439 would be about a few times 10 ^ { 5 } M _ { \odot } , its size about 1 kpc , and the dynamical mass M _ { dyn } > 5 \times 10 ^ { 7 } M _ { \odot } .
However , if GBT 1355+5439 is a dark Local Group object , the internal kinematics of the H i appears to be different from that of gas-dominated , almost dark galaxies of similar size .