  (Abridged). One of the most metal-deficient blue compact galaxies (BCGs) HS
0822+3542 (Z=1/34 Zsun), is also one of the nearest such objects. A trigger
mechanism for its current SF burst has remained unclear. We report the
discovery of a very blue ((B-V)tot=0.08 and (V-R)tot=0.14) LSB (mu_B^0 > 23.4
arcsec^-2) dwarf irregular (dIrr) galaxy, named SAO 0822+3545. Its small
relative velocity and projected distance of ~11 kpc from the BCG imply their
physical association. For this LSB galaxy, we present spectroscopic results,
total B,V,R magnitudes, the effective radii and surface brightness (SB), and we
describe its morphological properties. We compare the very blue colours of this
dwarf with PEGASE.2 models of the colour evolution of a Z=1/20 Zsun stellar
population, and combine this analysis with the data on the LSBD EW(Ha) values.
The models best describing all available observational data depend on the
relative fraction of massive stars in the IMF used. For a Salpeter IMF with Mup
= 120 Msun, the best model includes a "young" single stellar population (SSP)
with an age of ~10 Myr and an "old" SSP with the age of ~0.2--10 Gyr. The mass
ratio of the old to young components should be in the range of 10 to 30. The
role of interaction in triggering the galaxies major SF episodes during the
last ~100-200 Myr is discussed. For the BCG, based on the spectroscopy with the
6m telescope, we estimate the physical parameters of its SF region and present
the first evidence of an ionized gas supershell. This pair of dwarfs lies deep
within the nearby Lynx-Cancer void, with the nearest bright (L > L*) galaxies
at distances > 3 Mpc. This is probably one of the main factors responsible for
the unevolved state of HS 0822+3542.
