  "Cometary" Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by
off-center starbursts close to the end of an elongated dwarf irregular
(dI)-like host galaxy. This may either represent randomly enhanced star-forming
activity of a dI, or may be caused by a set of special properties of such
systems or their environment. For a first investigation of this issue, we
analyse the nearby iI,C BCDs Markarian 59 and Markarian 71. Using deep
ground-based spectrophotometric data and HST images, we derive physical
properties, structure and ages of the starburst regions and the underlying
stellar host galaxies. The metallicities show small scatter in the vicinity of
the star-forming regions and along the major axis of Mrk 59 which suggests
effective mixing of heavy elements on kpc scales. The surface brightness
profiles of the underlying host galaxies in either iI,C BCD show an exponential
decay with a central surface brightness and scale length that are intermediate
between typical iE/nE BCDs and dIs. Spectral population synthesis models in
combination with colour magnitude diagrams and colour profiles yield most
likely formation ages of ~2 Gyr for the host galaxies in both iI,C BCDs, with
upper limits of ~4 Gyr for Mrk 59 and ~3 Gyr for Mrk 71, i.e. significantly
lower than the typical age of several Gyr derived for the host galaxies of
iE/nE BCDs. These findings raise the question whether iI,C systems form a
distinct class within BCDs with respect to the age and structure of their
hosts, or whether they represent an evolutionary stage connecting young i0 BCDs
and "classical" iE/nE BCDs. Properties of analogous objects studied in the
local universe and at medium redshifts provide some support for this
evolutionary hypothesis.
