Among dwarf blue compact galaxies, I Zw 18 and II Zw 40 have continuum and emission-line spectra similar to those of giant H Ir regions in external galaxies. An analysis of the emission- line spectrum shows that the emissive gas is much hotter than normal galactic H Ir regions and that the heavy elements Z are highly underabundant unless they are in stages of ionization higher than Z2 +. A study of the thermal balance (i) confirms an underabundance of elements heavier than He by a factor of at least 10 compared to their cosmic values, and (ii) shows that most of the interstellar gas is ionized by hot stars, 4.0 >c 10 < Teff <% 5.5 >c 10 K. These stars must be massive young 0 stars, M > 30 M0. The He Ir A4686 line observed in emission in I Zw 18 can arise in the optically thin hot envelopes of radiation-driven winds of Of stars. These blue compaci galaxies cannot be old galactic systems with a flat mass distribution of newly formed stars. Subject headings: galaxies: stellar content - stars: abundances - stars: early-type - stars: emission-line
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J. Bergeron (1977) studied this question.