different mass fractions $M_{cl}/M_{SF}$ with respect to the mass involved in the SF region The crosses represent evolutionary timesteps of models, the arrows depict the direction of the evolutionary paths. The dashed straight line represents a simple model relation for purely secondary nitrogen production. [*For discussion see [@koe05].*]{} []{data-label="fig:NO"}](NO_OH_loop.pdf){width="10cm"}

Although the mass-metallicity relation also holds for dIrrs and even steepens its slope [@tre04], what can be interpreted by galactic mass loss and the corresponding lower effective yield [@gar02], the abundance ratios are unusual. As mentioned above, O/Fe reaches already solar values for subsolar oxygen or iron abundances. While this can be explained by a long SF timescale, another characteristic signature is that the ratio stays at about -1.6 to -1.5 with O abundances below 1/10 solar and with an increasing scatter with O (see Fig.\[fig:NO\]). Their regime of N/O–O/H values overlaps with those of regions in the outermost disk parts of spirals at about = 8.0 ... 8.5 [@zee98a].

In the 90th several authors have tried to model these observations by SF variations with gas loss through galactic winds under the assumptions that these dIrrs and blue compact DGs (BCDs) are young and experience their first epochs of SF (for 