ar X iv : a st ro - p h / 98 10 08 3 v 1 6 O ct 1 99 8 INFNFE - 15 - 98 Bounds on hep neutrinos

The excess of highest energy solar-neutrino events recently observed by Superkamiokande can be in principle explained by anomalously high hep-neutrino flux Φ ν (hep). Without using SSM calculations, from the solar lumi-nosity constraint we derive that Φ ν (hep)/S 13 cannot exceed the SSM estimate by more than a factor three. If one makes the additional hypothesis that hep neutrino production occurs where the 3 He concentration is at equilibrium, helioseismology gives an upper bound which is (less then) two times the SSM prediction. We argue that the anomalous hep-neutrino flux of order of that observed by Superkamiokande cannot be explained by astrophysics, but rather by a large production cross-section.