  It is of broad interest for galaxy formation theory to carry out a full
inventory of the numbers and properties of dwarf galaxies in the Local Volume,
both satellites and isolated ones. Ultra-deep imaging in wide areas of the sky
with small amateur telescopes can help to complete the census of these hitherto
unknown low surface brightness galaxies, which cannot be detected by the
current resolved stellar population and HI surveys. We report the discovery of
Donatiello I, a dwarf spheroidal galaxy located one degree from the star Mirach
(Beta And) in a deep image taken with an amateur telescope. The
color--magnitude diagram obtained from follow-up observations obtained with the
Gran Telescopio Canarias (La Palma, Spain) reveals that this system is beyond
the Local Group and is mainly composed of old stars. The absence of young stars
and HI emission in the ALFALFA survey are typical of quenched dwarf galaxies.
Our photometry suggests a distance modulus for this galaxy of (m-M)=27.6 +/-
0.2 (3.3 Mpc), although this distance cannot yet be established securely owing
to the crowding effects in our color--magnitude diagram. At this distance,
Donatiello I's absolute magnitude (M_v =-8.3), surface brightness (mu_v=26.5
mag arcsec^-2) and stellar content are similar to the "classical" Milky Way
companions Draco or Ursa Minor. The projected position and distance of
Donatiello I are consistent with being a dwarf satellite of the closest S0-type
galaxy NGC 404 ("Mirach's Ghost"). Alternatively, it could be one of the most
isolated quenched dwarf galaxies reported so far behind the Andromeda galaxy.
