THE STELLAR CONTENT OF NGC-3109

Deep B, V, and I CCD images are used to investigate the stellar content of a field along the major axis of the nearby dwarf galaxy NGC 3109. Stars spanning a wide range of ages are detected, and the youngest have ages between 10 and 30 million years. The width of the main sequence can be explained solely by observational uncertainties, indicating that star-to-star differences in reddening are small. The total line-of-sight reddening, derived from the main-sequence locus, is E(B-V) approximately 0.14. The distance modulus of NGC 3109 is rederived from published Cepheid photometry after assuming that the absorption measured in this field is representative of the galaxy in general, and the result is mu0 approximately 25.05. The luminosity function of upper main-sequence stars follows a well-defined power law with exponent 0.37 +/- 0.01. This is flatter than what is typically seen in other galaxies, and it is suggested that the shallow luminosity function may be the consequence of stochastic variations in the local star-forming history. Asymptotic giant branch (AGB) stars have also been detected. The AGB tip occurs near M(bol) approximately -7.0, indicating that an intermediate-age population is present. Moreover, the bolometric luminosity function of AGB stars is identical to that seen in the Small Magellanic Cloud, a galaxy with similar integrated absolute brightness. Finally, the locus of the red giant branch is very blue, indicating that the old and intermediate-age populations are extremely metal-poor, with [Fe/H] less-than-or-equal-to -1.6.