  Experimental determinations of the pp and pep fluxes have great
potentialities. We briefly review the reasons that make such measurements
privileged tests of neutrino properties. We discuss the predictions for these
fluxes given by four good solutions to the solar neutrino problem: small- and
large-angle MSW and Just-So oscillations into active neutrinos, and small-angle
MSW oscillations into sterile neutrinos. In addition, we examine the impact of
the planned Hellaz detector, which should measure separately the nu_e and nu_mu
fluxes in the pp energy window and the signal from the pep neutrinos, for
distinguishing among the different solutions and for determining the solar
central temperature.
