  We present the results of spectrophotometry and BVR CCD photometry, as well
as integrated HI radio measurements of a new blue compact galaxy (BCG) HS
0822+3542 with extremely low oxygen abundance: 12+log(O/H) = 7.35, or 1/36 of
solar value. The galaxy is the third most metal-deficient BCG after I Zw 18 and
SBS 0335-052. Its very high mass fraction of gas (~95% of all visible mass) and
blue colours of underlying nebulosity are also similar to those of SBS
0335-052. This suggests that HS 0822+3542 is one of the nearest and dimmest
galaxies experiencing a recently-started first star formation (SF) episode. Its
properties imply that for such galaxies there is a linear scaling of the main
parameters, at least for the baryon mass range (0.3-20)x10^8 M_sun. The total
mass estimate indicates that the galaxy is dynamically dominated by a dark
matter (DM) halo, which itself is one of the least massive for galaxies.
