For many years we are witnessing a lively debate on the existence and extent of convective overshooting, mainly in the cores of main-sequence stars. This is an important issue, since even a small amount of overshooting increases considerably the mass of the finally hydrogen exhausted core and lenghthens the main-sequence lifetime correspondingly. The available evolutionary calculations assume either moderate overshooting, d/Hp = 0.25, (d = overshooting distance, Hp = pressure scale height; Maeder & Meynet 1988) or strong overshooting, d/Hp ≈ 0.50 (Bertelli et al. 1986). Presently theory is unable to quantify the exact amount of overshooting, and one has to resort to empirical determinations.Recently, Stothers (1991) collected all available information from the literature on stellar parameters and evolutionary calculations and concluded that, within the errors, d/Hp = 0 is an acceptable result, with a conservative upper limit of d/Hp < 0.2. However, such an approach is hampered by observational errors (like distance or temperature uncertainties, rotation) that are difficult to quantify and that may mask any definitive result. Detailed investigations of detached binaries may help in this matter (Andersen et al. 1990) but the number of suitable binary systems is probably not very large.

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Abstract. Dwarf galaxies are ideal laboratories to study influential effects on galaxy evolution. In particular, their gas-rich variant with very active star formation, starbursting dwarf irregulars, shows chemical and structural signatures that lead unambiguously to the conclusion that they are standing in a vital contact with their surroundings. Gas infall cannot only trigger star formation but also allows for a reduction of the metal content. On the other hand, active star formation ignites numerous supernovae type II which accumulate and can produce a galactic wind. This again depletes the metals pushing them into a gas mixing cycle with different timescales, locally of about 10 Myrs, but an galactic scales of at least 1 Gyr. This paper illuminates the different processes like gas infall and outflow and their effects on the chemical evolution, the star formation, and the gas mixing in dwarf irregular galaxies.

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Investigations have been performed to understand the age of dwarf irregular galaxies from the point of view of their presently low chemical abundances and the enhanced star-formation rate. While for a few objects, like e.g. I Zw 18, their assumed relative youth can be verified by models taking a double-starburst behavior into account, those dlrrs with old underlying stellar populations have passed the regime of observed N/O-0 values within almost 2 Gyrs after their formation even with gas loss by means of supernova-driven galactic winds. Therefore alternative processes like gas infall must be invoked.

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