  We show that solar neutrino experiments set an upper limit of 7.8% (7.3%
including the recent KamLAND measurements) to the fraction of energy that the
Sun produces via the CNO fusion cycle, which is an order of magnitude
improvement upon the previous limit. New experiments are required to detect CNO
neutrinos corresponding to the 1.5% of the solar luminosity that the standard
solar model predicts is generated by the CNO cycle.
