The condition for the preservation of gauge invariance and unit flux q
uantization in a mixed charge (n and p type) superconducting system is
obtained. The result supports the high T-c tri-excitonic pair condens
ation model [K.W. Wong and R.L. Davidchack, submitted to Phys. Rev. Le
tt.]. This result also predicts the existence of a half-integer h/4e q
uantum flux in a tri-crystal junction.