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We present HI observations of the compact high-velocity cloud HVC289+33+251that was discovered by Putman et al. (2002). Observations with the 100-mEffelsberg telescope demonstrate that this cloud is still unresolved by the 9'beam of the Effelsberg telescope. The cloud shows a small line width of Deltav_FWHM = 4.9 km/s providing an upper limit to the kinetic temperature of the HIgas 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 AustraliaTelescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations thatare embedded in a common HI envelope. The HVC shows a faint tail, indicating anongoing ram-pressure interaction with an ambient low-density medium. A FWHMdiameter of theta = 4.4 arcmin makes this HVC the by far most compact HVC knowntill now. The observed parameters suggest that pressure stabilization by anambient medium is rather unlikely. At a distance of 150 kpc, the virial mass isby a factor of 5.6 higher than the observed gas mass -- consistent withHVC289+33+251 being one of the "missing" dark matter mini halos that werepredicted by cosmological LCDM simulations (e.g. Klypin et al. 1999; Moore etal. 1999). Comparable clouds in other groups of galaxies or even around theMilky Way are not detectable with the resolution and sensitivity of presentsurveys.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter

H I observations of an Ultra-Compact High-Velocity Cloud