  We present a ground-based optical spectroscopic and HST U, V, I photometric
study of the blue compact dwarf (BCD) galaxy Pox 186. It is found that the
emission of the low-surface brightness (LSB) component in Pox 186 at radii <
3'' (< 270 pc in linear scale) is mainly gaseous in origin. We detect Halpha
emission out to radii as large as 6''. At radii > 3'' the light of the LSB
component is contaminated by the emission of background galaxies complicating
the study of the outermost regions. The surface brightness distribution in the
LSB component can be approximated by an exponential law with a scale length
alpha < 120 pc. This places Pox 186 among the most compact dwarf galaxies
known. The derived alpha is likely to be an upper limit to the scale length of
the LSB component because of the strong contribution of the gaseous emission.
The oxygen abundance in the bright HII region derived from the 4.5m Multiple
Mirror Telescope (MMT) and 3.6m ESO telescope spectra are 12+log(O/H)=
7.76+/-0.02 and 7.74+/-0.01 (~ Zsun/15), respectively, in accordance with
previous determinations. The helium mass fractions found in this region are
Y=0.248+/-0.009 (MMT) and Y=0.248+/-0.004 (3.6m) suggesting a high primordial
helium abundance.
