  We present HI observations of the compact high-velocity cloud HVC289+33+251
that was discovered by Putman et al. (2002). Observations with the 100-m
Effelsberg telescope demonstrate that this cloud is still unresolved by the 9'
beam of the Effelsberg telescope. The cloud shows a small line width of Delta
v_FWHM = 4.9 km/s providing an upper limit to the kinetic temperature of the HI
gas of T_k < 532 K. The total observed flux indicates an HI mass of M(HI) =
5.66 10^4 M_sun [d/150kpc]^2. Follow-up HI observations using the Australia
Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that
are embedded in a common HI envelope. The HVC shows a faint tail, indicating an
ongoing ram-pressure interaction with an ambient low-density medium. A FWHM
diameter of theta = 4.4 arcmin makes this HVC the by far most compact HVC known
till now. The observed parameters suggest that pressure stabilization by an
ambient medium is rather unlikely. At a distance of 150 kpc, the virial mass is
by a factor of 5.6 higher than the observed gas mass -- consistent with
HVC289+33+251 being one of the "missing" dark matter mini halos that were
predicted by cosmological LCDM simulations (e.g. Klypin et al. 1999; Moore et
al. 1999). Comparable clouds in other groups of galaxies or even around the
Milky Way are not detectable with the resolution and sensitivity of present
surveys.
