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In order to recognize environmental effects on the evolution of dwarf galaxies in clusters of galaxies, it is first necessary to quantify the properties of objects which have evolved in relative isolation. With oxygen abundance as the gauge of metallicity, two key diagnostics of the evolution of dwarf irregular galaxies in the field are re-examined: the metallicity-luminosity relationship and the metallicity-gas fraction relationship. Gas fractions are evaluated from the masses of luminous components only, i.e., constituents of the nucleogenetic pool. Results from new optical spectroscopy obtained for H II regions in five dwarf irregular galaxies in the Local Volume are incorporated into a new analysis of field dwarfs with [O III]λ4363 detections and good distances. The updated fit to the metallicity-luminosity relationship is consistent with results reported in the literature. The fit to the metallicity-gas fraction relation shows an excellent correlation consistent with expectations of the simple "closed box" model of chemical evolution. The simplest explanation consistent with the data is that flow rates are zero, although the observations allow for the possibility of modest flows. The derived oxygen yield is one-quarter of the value for the solar neighbourhood. 6 Low-luminosity ellipticals or LLEs (e.g., M32) are structurally similar to giant ellipticals and exhibit a trend of decreasing surface brightness with increasing luminosity.

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Abstract. We present optical spectroscopy of H ii regions in the Local Group galaxy IC 10 and UBV R photometry of foreground stars in three fields towards this galaxy. From these data, we find that the foreground reddening due to the Milky Way is E(B − V ) = 0.77 ± 0.07 mag. We find that IC 10 contains considerable internal dust, which qualitatively explains the variety of reddening values found by studies of its different stellar populations. Based upon our foreground reddening, IC 10 has intrinsic photometric properties like those of a blue compact dwarf galaxy, and not those of a dwarf irregular. This result is consistent with much evidence that IC 10 is in the throes of a starburst that began at least 10 Myr ago. We also report the discovery of a new WR star in the H ii region HL111c.Key words. galaxies: individual: IC 10 -galaxies: local group -galaxies: fundamental parameters -galaxies: starburst -galaxies: dwarf IntroductionIC 10 has long been recognized as a peculiar object (Hubble 1936), and is now considered the Local Group's representative of a starburst galaxy (van den Bergh 2000). As we will show, IC 10 may very well be the nearest example of a blue compact dwarf galaxy (BCD). Certainly, its surface brightness is similar to that seen in BCDs once we account for the foreground reddening. Among Local Group dwarf galaxies, IC 10 has the highest surface density of WR stars and the highest current rate of star formation (Mateo 1998). The presence of so many WR stars and the high Hα luminosity emphasize that IC 10 is undergoing a strong burst of star formation that began at Send offprint requests to: M. G. Richer, e-mail: richer@astrosen.unam.mx least 10 Myr ago. Observations of 21-cm emission from H i reveal that IC 10 consists of an inner disk embedded in an extended, complex, counter-rotating envelope (Shostak & Skillman 1989), and leads Wilcots & Miller (1998) to conclude that IC 10 is still in its formative stage. These H i observations emphasize the youth of the current star formation episode, for there is a notable lack of interstellar medium structures that are attributable to supernovae (Wilcots & Miller 1998). Several studies of the stellar populations in IC 10 have revealed the existence of young, intermediate-age, and old stellar populations (Massey & Armandroff 1995;Sakai et al. 1999;Borissova et al. 2000). However, with the exception of the very recent star formation, very little is known of the history of star formation in IC 10, and nothing is known of the star formation history outside of the star-forming region.Article published by EDP Sciences and available at http://www.aanda.org or http://dx