  Isotopic ratios of C, N, Si, and trace heavy elements in presolar SiC grains
from meteorites provide crucial constraints to nucleosynthesis. A long-debated
issue is the origin of the so-called A+B grains, as for them no stellar
progenitor has so far been clearly identified on observational grounds. We
report the first spectroscopic measurements of 14N/15N ratios in Galactic
carbon stars of different spectral types and show that J- and some SC-type
stars might produce A+B grains, even for 15N enrichments previously attributed
to novae. We also show that most mainstream (MS) grains are compatible with the
composition of N-type stars, but might also descend, in some cases, from SC
stars. From the theoretical point of view, no astrophysical scenario can
explain the C and N isotopic ratios of SC, J and N-type carbon stars together,
as well as those of many grains produced by them. This poses urgent questions
to stellar physics.
