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We present the results from the analysis of optical spectra of 31Halpha-selected regions in the extended UV (XUV) disks of M83 (NGC5236) andNGC4625 recently discovered by GALEX. The spectra were obtained using IMACS atLas Campanas Observatory 6.5m Magellan I telescope and COSMIC at the Palomar200-inch telescope, respectively for M83 and NGC4625. The line ratios measuredindicate nebular oxygen abundances (derived from the R23 parameter) of theorder of Zsun/5-Zsun/10. For most emission-line regions analyzed the linefluxes and ratios measured are best reproduced by models of photoionization bysingle stars with masses in the range 20-40 Msun and oxygen abundancescomparable to those derived from the R23 parameter. We find indications for arelatively high N/O abundance ratio in the XUV disk of M83. Although themetallicities derived imply that these are not the first stars formed in theXUV disks, such a level of enrichment could be reached in young spiral disksonly 1 Gyr after these first stars would have formed. The amount of gas in theXUV disks allow maintaining the current level of star formation for at least afew Gyr.Comment: 52 pages, 8 tables, 7 figures, accepted for publication in Ap

Chemical and Photometric Evolution of Extended Ultraviolet Disks: Optical Spectroscopy of M83 (NGC 5236) and NGC 4625