HS 0822+3542 - a new nearby extremely metal-poor galaxy

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 1/3 6 of solar value. The galaxy is the third most metal-deficient BCG after I Zw is and SBS 0335-052. Its very high mass fraction of gas (approximate to 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 o f the nearest and dimmest galaxies experiencing a recently-started first st ar formation (SF) episode. Its properties imply that for such galaxies ther e is a linear scaling of the main parameters, at least for the baryon mass range (0.3-20)x10(8) M-circle dot. The total mass estimate indicates that t he galaxy is dynamically dominated by a dark matter (DM) halo, which itself is one of the least massive for galaxies.