  Neutrinos produced in the Sun by electron capture reactions on $^{13}{\rm
N}$, $^{15}{\rm O}$ and $^{17}{\rm F}$, to which we refer as ecCNO neutrinos,
are not usually considered in solar neutrino analysis since the expected fluxes
are extremely low. The experimental determination of this sub-dominant
component of the solar neutrino flux is very difficult but could be rewarding
since it provides a determination of the metallic content of the solar core
and, moreover, probes the solar neutrino survival probability in the transition
region at $E_\nu\sim 2.5\,{\rm MeV}$. In this letter, we suggest that this
difficult measure could be at reach for future gigantic ultra-pure liquid
scintillator detectors, such as LENA.
