New southern blue compact dwarf galaxies in the 2dF Galaxy redshift survey

Aims. Aiming to find new extremely metal-deficient star-forming galaxies we extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100 K Data Release 14 emission-line galaxies with relatively strong [O III] lambda 4363 emission. Methods. Spectroscopic and photometric studies of this sample and, in addition, of 7 Tololo and 2 UM galaxies were performed on the basis of observations with the ESO 3.6m telescope. Results. All sample galaxies qualify with respect to their photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies. Additionally, they show a good overlap with a comparison sample of similar to 100 well-studied emission-line galaxies on the 12 + log (O/H) vs. log(Ne/O), log(Ar/O) and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the discovery of two new extremely metal-deficient BCDs with an oxygen abundance 12 + log (O/H) <= 7.6 and of another seven galaxies with 12 + log (O/H) less than or similar to 7.8. Furthermore, we confirm previous oxygen abundance determinations for the BCDs Tol 1304- 353, Tol 2146-391, UM 559 and UM 570 to be 12 + log (O/H) <= 7.8.