  If heavy neutrinos with mass $m_{\nu_{H}}\geq$2$ m_e $ are produced in the
Sun via the decay ${^8\rm{B}} \rightarrow {^8\rm{Be}} + e^+ + \nu_H$ in a side
branch of pp-chain, they would undergo the observable decay into an electron, a
positron and a light neutrino $\nu_{H}\rightarrow\nu_{L}+e^++e^-$. In the
present work Borexino data are used to set a bound on the existence of such
decays. We constrain the mixing of a heavy neutrino with mass 1.5 MeV $\leq
m_{\nu_{H}} \le$ 14 MeV to be $|U_{eH}|^2\leq (10^{-3}-4\times10^{-6})$
respectively. These are tighter limits on the mixing parameters than obtained
in previous experiments at nuclear reactors and accelerators.
