a detailed review see [@hens99]). Stellar population studies contradict this youth hypothesis, so that another process must be invoked. Since these objects are embedded into envelops and are suggested to suffer gas infall as manifested e.g. in NGC 1569 (see above, [@sti02; @mue05]), the influence of metal-poor gas infall into an old galaxy with continuous SF on particular abundance patterns were exploited by Koeppen & Hensler [@koe05]. With the reasonable assumption that the fraction of infalling-to-existing gas mass increases with decreasing galaxy mass, their results could match not only the observational regime of BCDs in the \[\]-space but also explain the shark-fin shape of observational data distribution [@koe05].

Fig.\[fig:NO\] demonstrates how self-enriched galaxies which have reached the secondary nitrogen-track already within 2-3 Gyrs of their evolution are thrown back in O abundance by gas infall while N/O stays the same. After a time delay depending on the mass fraction of infalling gas to that existing already within the SF site, along a loop-like evolutionary paths in the \[\]-diagram the ISM abundances reach again the starting point. In summary, one can state that old dIrrs [*mutate*]{} temporarily to youngly appearing examples with respect to their gas abundances.

Dwarf elliptical galaxies {#dEs}
