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We present the results of a complex study of the low surface brightness dwarf (LSBD) gas‐rich galaxies J0723+3621, J0737+4724 and J0852+1350, which reside in the nearby Lynx–Cancer void. Their ratios  M (H  i )/ L  B , according to H  i  data obtained with the Nançay Radio Telescope (NRT), are respectively ∼3.9, ∼2 and ∼2.6. For the two latter galaxies, we derived an oxygen abundance corresponding to the value of 12+log (O/H)  7.3, using spectra from the Russian 6‐m telescope (BTA) and the Sloan Digital Sky Survey (SDSS) data base. We found two additional blue LSBDs, J0723+3622 and J0852+1351, which appear to be physical companions of J0723+3621 and J0852+1350 situated at projected distances of ∼12–13 kpc. The companion relative velocities, derived from the BTA spectra, are Δ V =+89 km s −1  and +30 km s −1  respectively. The geometry and relative orientation of orbits and spins in these pairs indicate, respectively, prograde and polar encounters for J0723+3621 and J0852+1350. The NRT H  i  profiles of J0723+3621 and J0723+3622 indicate a sizeable gas flow in this system. The SDSS  u ,  g ,  r ,  i  images of the five dwarfs are used to derive photometric parameters and exponential or Sersic disc model fits. For three of them, the ( u − g ), ( g − r ), ( r − i ) colours of the outer parts, when compared with PEGASE evolutionary tracks, provide evidence for the dominance of old stellar populations with ages of  T ∼ (8–10) ± 3 Gyr. For J0723+3622 and J0737+4724 the outer region colours appear rather blue, implying ages of the oldest visible stars of  Gyr. The new LSB galaxies complement the list of known most metal‐poor and ‘unevolved’ dwarfs in this void, including DDO 68, SDSS J0812+4836, SDSS J0926+3343 and SAO 0822+3545. This unique concentration of ‘unevolved’ dwarf galaxies in a small cell of the nearby Universe implies a physical relationship between slow galaxy evolution and a void‐type global environment. We also compare the baryonic content of these LSBDs with predictions from the most updated cosmological simulations.

Study of galaxies in the Lynx–Cancer void – III. New extreme low surface brightness dwarf galaxies