  We perform near ultraviolet ACS prism spectroscopy of 11 Small Magellanic
Cloud (SMC) planetary nebulae (PNe) with the main aim of deriving the abundance
of carbon. The analysis of the ACS spectra provide reliable atomic carbon
abundances for all but a couple of our targets; ionic C^(2+) abundances are
calculated for all target PNe. With the present paper we more than double the
number of SMC PNe with known carbon abundances, providing a good database to
study the elemental evolution in low- and intermediate-mass stars at low
metallicity. We study carbon abundances of Magellanic Cloud PNe in the
framework of stellar evolution models and the elemental yields. Constraining
SMC and LMC stellar evolutionary models is now possible with the present data,
through the comparison of the final yields calculated and the CNO abundances
observed. We found that SMC PNe are almost exclusively carbon rich, and that
for the most part they have not undergone the hot-bottom burning phase,
contrary to half of the studied LMC PNe. The yields from stellar evolutionary
models with LMC and SMC metallicities broadly agree with the observations. In
particular, evolutionary yields for M$_{\rm to}<3.5 {\rm M}_{\odot}$ well
encompass the abundances of round and elliptical PNe in the SMC. We found that
the carbon emission lines are major coolants for SMC PNe, more so than in their
LMC counterparts, indicating that metallicity has an effect on the physics of
PNe, as predicted by Stanghellini et al. (2003).
