A critical study of the properties of groups of galaxies can be done only once a complete census of group members is available .
Despite extensive surveys , even nearby groups can lead to surprises .
Indeed we report the discovery of a previously unnoticed member of the Centaurus A Group , NGC 5011C .
While the galaxy is a well known stellar system listed with a NGC number its true identity remained hidden because of coordinate confusion and wrong redshifts in the literature .
NGC 5011C attracted our attention since , at a putative distance of 45.3 Mpc , it would be a peculiar object having a very low surface brightness typical of a dwarf galaxy , and at the same time having the size of an early-type spiral or S0 galaxy .
To confirm or reject this peculiarity , our immediate objective was to have the first reliable measurement of its recession velocity .
The observations were carried out with EFOSC2 at the 3.6m ESO telescope , and the spectra were obtained with the instrument in long-slit mode .
The redshifts of both NGC 5011C and its neighbor NGC 5011B were computed by cross-correlating their spectra with that of a radial velocity standard star .
We found that NGC 5011C has indeed a low redshift of v _ { \odot } = 647 \pm 96 { km sec } ^ { -1 } and thus is a nearby dwarf galaxy rather than a member of the distant Centaurus cluster as believed for the past 23 years .
Rough distance estimates based on photometric parameters also favor this scenario .
As a byproduct of our study we update the redshift for NGC 5011B at v _ { \odot } = 3227 \pm 50 { km sec } ^ { -1 } .
Applying population synthesis techniques , we find that NGC 5011B has a luminosity-weighted age of 4 \pm 1 Gyr and a solar metallicity , and that the luminosity-weighted age and metallicity of NGC 5011C are 0.9 \pm 0.1 Gyr and 1 / 5 solar .
Finally we estimate a stellar mass of NGC 5011C comparable to that of dwarf spheroidal galaxies in the Local Group .