  We have derived oxygen and nitrogen abundances of a sample of late-type, low
surface brightness (LSB) galaxies found in the Sloan Digital Sky Survey (SDSS).
Furthermore, we have computed a large grid (5000 models) of chemical evolution
models (CEMs) testing various time-scales for infall, baryon densities and
several power-law initial mass functions (IMFs) as well. Because of the rather
stable N/O-trends found both in CEMs (for a given IMF) and in observations, we
find that the hypotheses that LSB galaxies have stellar populations dominated
by low-mass stars, i.e., very bottom-heavy IMFs (see Lee et al. 2004), can be
ruled out. Such models predict much too high N/O-ratios and generally too low
O/H-ratios. We also conclude that LSB galaxies probably have the same ages as
their high surface brightness counterparts, although the global rate of star
formation must be considerably lower in these galaxies.
