  We study quintessence models with a constant (effective) equation of state.
It is investigated whether such quintessence models are consistent with a
negative spatial curvature of the Universe with respect to the anisotropy of
the cosmic microwave background radiation measured by the WMAP mission. If the
reionization is negligibly small, it is found that such models with negative
curvature are admissible due to a geometrical degeneracy. However, a very high
optical depth tau to the surface of last scattering, as indicated by the
polarization measurements of WMAP, would rule out such models.
