  (Abridged) As part of an ongoing effort to study the stellar nuclei of very
late-type, bulge-less spirals, we present results from a high-resolution
spectroscopic survey of nine such nuclear star clusters, undertaken with
VLT/UVES. We fit the spectra with population synthesis models and measure
Lick-type indices to determine mean luminosity-weighted ages, which range from
4.1*10^7 to 1.1*10^10 years and are insensitive to assumed metallicity or
internal extinction. The average metallicity of nuclear clusters in late-type
spirals is slightly sub-solar (<Z> = 0.015) but shows significant scatter. The
fits also show that the nuclear cluster spectra are best described by a mix of
several generations of stars. This is supported by the fact that only models
with composite stellar populations yield mass-to-light ratios that match those
obtained from dynamical measurements. The last star formation episode was on
average 34 Myr ago, while all clusters experienced some star formation in the
last 100 Myr. We thus conclude that the nuclear clusters undergo repeated
episodes of star formation. The robustness with respect to possible
contamination from the underlying galaxy disk is demonstrated by comparison to
spectra obtained with HST/STIS. Combining these results with those from Walcher
et al. (2005), we have thus shown that the stellar nuclei of these bulge-less
galaxies are massive and dense star clusters that form stars recurrently until
the present day. This unique set of properties is likely due to the central
location of these clusters in their host galaxies.
