One of the most metal-deficient blue compact galaxies ( BCGs ) HS 0822+3542 ( Z = 1 / 34 ~ { } Z _ { \odot } ) , is also one of the nearest such objects ( D \sim 11 Mpc ) .
It is in addition well isolated from known bright galaxies .
A trigger mechanism for its current star-formation ( SF ) burst has thus remained unclear .
We report the discovery of a very blue ( ( B - V ) ^ { 0 } _ { tot } = 0.08 and ( V - R ) ^ { 0 } _ { tot } = 0.14 ) low surface brightness ( LSB ) ( \mu _ { B } ^ { 0 } \gtrsim 23 \aas@@fstack { m } 4 arcsec ^ { -2 } ) dwarf irregular ( dIrr ) galaxy , which we have named SAO 0822+3545 .
Its small relative velocity and projected distance of only \sim 11 kpc from the BCG imply their physical association .
For this LSB dIrr galaxy , we present spectroscopic results , total B,V,R 
magnitudes , and 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 ~ { } Z _ { \odot } stellar population , and combine this analysis with the data on the LSBD EW ( H \alpha ) 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 M _ { up } = 120 M _ { \odot } , the best model includes a “ young ” single stellar population ( SSP ) with an age of \sim 10 Myr and an “ old ” SSP with the age of \sim 0.2–10 Gyr .
The mass ratio of the old to young components should be in the range of 10 to 30 .
If the age of the old component is more than \sim 1 Gyr , an additional coeval component of “ intermediate ” age ( \sim 100 Myr ) with a mass comparable to that of the “ young ” population , although not required , provided a good fit to the current data .
For the two options of a model IMF biased toward the low-mass end , the best match of the observed BVR and EW ( H \alpha ) is for continuous star-formation rate ( SFR ) single-component models , with SF durations in the range of \sim 0.1 to \sim 1 Gyr .
However , only a longer time-scale SF gives the stellar mass , compatible with the LSB galaxy mass estimates .
Nevertheless , such a scenario would be inconsistent with the recent encounter of these two dwarfs .
The role of interaction between the LSBD and BCG HS 0822+3542 in triggering their major SF episodes during the last \sim 100–200 Myr is emphasized and discussed .
For the BCG , based on the results of new spectroscopy with the Russian 6 m 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 .