  We present the star formation history of UGCA 290, a galaxy with properties
intermediate between Blue Compact Dwarfs and Dwarf Irregulars. This galaxy is
particularly interesting because its young stellar population is extremely
similar to that of the well-studied type iE Blue Compact Dwarf VII Zw 403,
despite its different spatial morphology and old stellar content. Our Hubble
Space Telescope/Wide Field Planetary Camera 2 single-star photometry for UGCA
290 extends over nine magnitudes in I, and allows a detailed study of its star
formation history. Using synthetic color-magnitude diagrams, we show that the
recent "burst" which gives this galaxy its BCD status is a moderate enhancement
in star formation which lasted for approximately 20 Myr, at a rate about ten
times above its previous rate. The star formation history for most of the
previous billion years is consistent with a constant rate, although
enhancements as large as the current one are possible at times ealier than 400
Myr ago. We estimate that the total mass converted into stars in UGCA 290 more
than one billion years ago is about three times the astrated mass since that
time. The initial mass function is consistent with a Salpeter slope, and the
stellar metallicity is bracketed by Z_sun/50 and Z_sun/5, with evidence for
metallicity evolution. Similar results for the star formation history over the
past 600 Myr apply to VII Zw 403. Our main result is that despite the
traditional picture of BCDs, the current bursts in these two galaxies are
neither remarkably intense nor short-lived, and that most of their star
formation occured more than a billion years ago.
