SBS 0335-052E+W: deep VLT/FORS plus UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies

Context. 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. Aims. Our aim is to derive element abundances in different H II regions of this unique system of galaxies and to study spatial abundance variations. Methods. The electron temperature T(e) (O III) in all H II regions, except for one, is derived from the [O III] lambda 4363/(lambda 4959+lambda 5007) flux ratio. We determine ionic abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon and iron. The empirical relations for ionization correction factors are used to derive total abundances of these elements. Results. The oxygen abundance in the brighter eastern galaxy varies in the range 7.11 to 7.32 in different H II regions supporting previous findings and suggesting the presence of oxygen abundance variations on spatial scales of similar to 1-2 kpc. Good seeing during FORS observations allowed us to extract spectra of four H II regions in SBS 0335-052W. The oxygen abundance in the brightest region No. 1 of SBS 0335-052W is 7.22 +/- 0.07, consistent with previous determinations. Three other H II regions are much more metal-poor with an unprecedently low oxygen abundance of 12 + log O/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, supporting earlier conclusions that SBS 0335-052W is the lowest-metallicity emission-line galaxy known. Helium abundances derived for the brightest H II 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 He I emissivities. The ratios of neon and sulfur to oxygen abundance are similar to the respective ratios obtained for other emission-line galaxies. On the other hand, the chlorine-to-oxygen abundance ratio in SBS 0335-052E is lower, while the argon-to-oxygen abundance ratio is higher than those in other low-metallicity galaxies. The Fe/O abundance ratios in different regions of SBS 0335-052E are among the highest for emission-line galaxies implying that iron is almost entirely not depleted onto dust grains despite dust being detected in this galaxy in earlier ISO and Spitzer observations. 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.