Auto-consistent metallicity and star formation history of the nearest
  blue compact dwarf galaxy NGC 6789
  We present a detailed auto-consistent study of the nearest blue compact dwarf
galaxy NGC 6789 by means of optical and UV archive photometry data and optical
long-slit ISIS-WHT spectroscopy observations of the five brightest star-forming
knots. The analysis of the spectra in all knots allowed the derivation of ionic
chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures
of both the high- and low-excitation electron temperatures, leading to the
conclusion that NGC 6789 is chemically homogeneous with low values of the
abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the
same time higher values of the nitrogen-to-oxygen ratio than expected for its
metal regime.
  We used archival HST/WFPC2 F555W and F814W observations of NGC 6789 to
perform a photometric study of the colour-magnitude diagram (CMD) of the
resolved stellar populations and derive its star formation history (SFH), which
is compatible with the presence of different young and old stellar populations
whose metallicities do not necessarily increase with age. We fit the observed
optical spectrum in all the five knots using the STARLIGHT code and a
combination of single stellar populations following the SFH obtained from the
CMD. We compare the resulting stellar masses and the relative fractions of the
ionising populations with a non-constrained SFH case. The properties of the
younger populations were obtained using CLOUDY photoionisation models, giving
similar ages in all the knots in the range 3-6 Myr and the estimation of the
dust absorption factor, which correlates with the observed GALEX FUV-NUV colour
indices. The total photometric extinction and dust-absorption corrected
H\alpha\ fluxes were finally used to derive the star formation rates.
