  Aims. We determine masses and mass-to-light ratios of five ultra-compact
dwarf galaxies and one dwarf elliptical nucleus in the Fornax cluster from high
resolution spectroscopy. Methods. Velocity dispersions were derived from
selected wavelength regions using a direct-fitting method. To estimate the
masses of the UCDs a new modelling program has been developed that allows a
choice of different representations of the surface brightness profile (i.e.
Nuker, Sersic or King laws) and corrects the observed velocity dispersions for
observational parameters (i.e. seeing, slit size). Results. The observed
velocity dispersions range between 22 and 30 km/s. The resulting masses are
between 1.8 and 9.5x10^7M_sun. These, as well as the central and global
projected velocity dispersions, were derived from the generalized King model
which turned out to give the most stable results. The masses of two UCDs, that
are best fitted by a two-component profile, were derived from a combined
King+Sersic model. The mass-to-light ratios of the Fornax UCDs range between 3
and 5 (M/L_V)_sun. Within 1-2 half-mass radii dark matter is not dominating
UCDs. Conclusions. We show that the mass-to-light ratios of UCDs in Fornax are
consistent with those expected for pure stellar populations. Thus UCDs seem to
be the result of cluster formation processes within galaxies rather than being
compact dark matter dominated substructures themselves. Whether UCDs gained
their mass in super-star cluster complexes of mergers or in nuclear star
cluster formation processes remains an open question. It appears, however,
clear that star clusters more massive than about 5\times10^6M_sun exhibit a
more complex formation history than the less massive `ordinary' globular
clusters.
