  The aim of the paper is to determine abundances in a group of PNe with
uniform morphology. The PNe discussed are circular excited by rather
low-temperature central stars. The relation between abundance and evolution is
discussed. The mid-infrared spectra of NGC1535, NGC6629, He2-108 and Tc1 taken
with the Spitzer Space Telescope are presented. These spectra are combined with
IUE and visual spectra to obtain complete extinction-corrected spectra from
which the abundances are determined. These abundances are more accurate for
several reasons, the most important is that the inclusion of the far infrared
spectra increases the number of observed ions and makes it possible to include
the nebular temperature gradient in the abundance calculation. The abundances
of these PNe are compared to those found in five other PNe of similar
properties and are further compared with predictions of evolutionary models.
From this comparison we conclude that these PNe originated from low mass stars,
probably between 1 and 2.5 solar masses and at present have core masses between
0.56 and 0.63 solar masses. A consistent description of the evolution of this
class of PNe is found that agrees with the predictions of the present nebular
abundances, the individual masses and the luminosities of these PNe. The
distances to these nebulae can be found as well.
