2000A&A...359..493M

We present results of narrow-band (Hα and adjacent continuum) and broad-band (U, B and V) optical CCD imaging together with high- and intermediate-resolution optical spectroscopy for a sample dwarf and/or irregular Wolf-Rayet (WR) galaxies with absolute B magnitudes in the range -14 to -22mag, taken from the catalogue of Conti (1991ApJ...377..115C). We find that the recent star formation processes in the galaxies of the sample are distributed in different knots. These knots are H ii regions probably ionized by so-called super star clusters (or aggregates of them) found in space observations of WR and interacting galaxies. A comparative study of the U-B colour and the -W(Hα) of the different star-forming knots of the galaxies indicates that these two magnitudes give consistent age estimates. However, the B-V colour give comparatively greater ages, which can be explained by the presence of underlying stellar populations in many of the objects. This is confirmed by the presence of a much more extended and diffuse morphology (in some cases with a disc shape) in broad-band compared to Hα images. Our study has also revealed that a substantial fraction of irregular and dwarf WR galaxies at first classified as isolated objects, may in fact be interacting or merging with other low surface brightness companions that escaped detection in previous studies. These interaction processes could be the cause of the triggering of the strong star formation we are now seeing in many of the objects. The Hα morphology of the galaxies indicates that the presence of bubble-like and low surface brightness filamentary structures is a rather common characteristic of these kinds of objects. Spectroscopic observations reported in this and previous papers confirm the presence of high-velocity asymmetric flows that extend to the outer zones in several galaxies.