  Relic neutrinos from the early universe are predicted to have a relatively
large number density, but extremely low energies. Hence, the only possible
interaction proceeds via neutrino capture on beta-decaying nuclei. In case
relic neutrinos are captured by beta-decaying nuclei in the Sun, the neutrinos
normally emerging from the decay of these nuclei will be missing from the
overall number of solar neutrinos registered in neutrino experiments. Within
the Sun, ${}^8\mathrm{B}$ and three nuclei from the CNO cycle are found to be
suitable for this kind of process. Their cross section as well as the possible
impact on the observed number of solar neutrinos are discussed. Assuming
standard neutrino oscillations, no deviations of neutrino flux measurements
from the predictions of the solar standard model are observed and upper limits
are derived accordingly.
