ELEMENT ENRICHMENT AND STELLAR POPULATIONS OF GAS-RICH LOW SURFACE BRIGHTNESS DWARF GALAXIES

The results of evolutionary studies of 15 gas-rich, low surface bright ness dwarf galaxies (LSBDGs) are presented. The LSBDGs are distinguish ed by high values of hydrogen-mass-to-optical-luminosity and the prese nce of extended H I envelopes, While these properties suggest a relati vely inefficient star formation history, these objects are not ''young '' systems. Based on UBVR colors, the dominant stellar populations are at least 2-4 Gyr old. The derived oxygen abundances are low and are i n agreement with the previously known metallicity-luminosity relation for low mass objects, Similar to other studies of relative enrichments of heavy elements in low mass systems, the observed N/O ratio suggest s a primary origin of nitrogen in low abundance systems. While outflow or inflow of gas cannot be ruled out (particularly in such gas-rich s ystems), closed box estimates with normal nucleosynthetic yields are c onsistent with the observed metallicities. (C) 1997 The American Astro nomical Society. [S0004-6256(97)04612-8].