Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies - II. HS 1442+4250

We present broad-band V and I imaging and long-slit spectroscopy in the optical range lambdalambda3600-7500 Angstrom of the dwarf irregular galaxy HS 1442+4250. The oxygen abundance 12 + log (O/H) = 7.63 +/- 0.02 (Z = Z(circle dot)19)(star) in the brightest H II region of HS 1442+4250 places the galaxy among the most metal-deficient emission-line galaxies. The low metallicity and blue colour (V-I) similar to0.4 mag of the low-surface-brightness (LSB) component make HS 1442+4250 a likely rare young dwarf galaxy candidate. We use four methods to estimate the stellar population age in the LSB component of HS 1442+4250. Different star formation histories are considered. The equivalent widths of hydrogen Halpha and Hbeta emission lines, and of hydrogen Hgamma and Hdelta absorption lines, the spectral energy distribution and the observed (V-I) colours of the LSB regions are reproduced quite well by models with only young and intermediate-age stellar populations. By contrast, the observational data cannot be reproduced by a stellar population formed continuously with a constant star formation rate in the age range from 0 to greater than or equal to 2 Gyr. While a faint old stellar population in HS 1442+4250 with an age greater than or equal to2 Gyr is not excluded, we find no evidence for such a population from the present data.