SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the
  lowest-metallicity blue compact dwarf galaxies
  (abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.
