We have identified a sample of 41 low-mass high–oxygen abundance outliers from the mass–metallicity relation of star-forming galaxies measured by Tremonti et al .
( 35 ) .
These galaxies , which have 8.6 < 12 + \log ( \mbox { O } / \mbox { H } ) < 9.3 over a range of -14.4 > M _ { B } > -19.1 and 7.4 < \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) < 10 , are surprisingly non-pathological .
They have typical specific star formation rates , are fairly isolated and , with few exceptions , have no obvious companions .
Morphologically , they are similar to dwarf spheroidal or dwarf elliptical galaxies .
We predict that their observed high oxygen abundances are due to relatively low gas fractions , concluding that these are transitional dwarf galaxies nearing the end of their star formation activity .