(Abridged) We present the results of the 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(HI)/L_B, according to HI data obtai
ned with the NRT, are respectively ~3.9, ~2, ~2.6. For the two latter galaxies, we derived oxygen abundance corresponding to the value of 12+log(O/H) <~7.3, using spectra from the Russian 6m telescope and from the SDSS database. We found two additional blue LSB dwarfs, J0723+3622 and J0852+1351, which appear to be physical companions of J0723+3621 and J0852+1350 situated at the projected distances of ~12--13 kpc. The companion relative velocities, derived from the BTA spectra, are dV = +89 km/s and +30 km/s respectively. The geometry and the relative orientation of orbits and spins in these pairs indicate, respectively, prograde and polar encounters for J0723+3621 and J0852+1350. The NRT HI profiles of J0723+3621 and J0723+3622 indicate a sizable gas flow in this system. The SDSS u,g,r,i images of the five dwarfs are used to derive the photometric parameters and the exponential or Sersic disc model fits. For three of them, the (u-g),(g-r),(r-i) colours of the outer parts, being compared with the PEGASE evolutionary tracks, evidence for the dominance of the 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 the ages of the oldest visible stars of T <~1-3 Gyr. The new LSB galaxies complement the list of the known most metal-poor and `unevolved dwarfs in this void, including DDO 68, SDSS J0926+3343 and others. This unique concentration of unevolved dwarf galaxies in a small cell of the nearby Universe implies a physical relationship between the slow galaxy evolution and the void-type global environment.
And IV is a low-surface brightness (LSB) dwarf galaxy at the distance of 6.1 Mpc, projecting close to M 31. In this paper the results of spectroscopy of And IV the two brightest HII regions with the SAO 6-m telescope (BTA) are presented. In both of t
hem the faint line [OIII]4363 was detected that allowed us to determine their O/H by the classical T_e method. Their values of 12+log(O/H) are equal to 7.49+-0.06 and 7.55+-0.23, respectively. The comparison of these direct O/H determinations with the two most reliable semi-empirical and empirical methods shows their good consistency. For And IV absolute blue magnitude of M_B=-12.6, our value of O/H corresponds well to the `standard relation between O/H and L_B for dwarf irregular galaxies (DIGs). And IV appears to be a new representative of the extremely metal-deficient gas-rich galaxies in the Local Volume. The very large range of M(HI) for LSB galaxies with close metallicities and luminosities indicates that the simple models of LSBG chemical evolution are too limited to predict such striking diversity.
