First determination of the CP content of [formula omitted] and [formula omitted].

Quantum-correlated ψ ( 3770 ) → D D ¯ decays collected by the CLEO-c experiment are used to perform first measurements of F + , the fractional CP -even content of the self-conjugate decays D → π + π − π 0 and D → K + K − π 0 . Values of 0.968 ± 0.017 ± 0.006 and 0.731 ± 0.058 ± 0.021 are obtained for π + π − π 0 and K + K − π 0 , respectively. It is demonstrated how modes of this sort can be cleanly included in measurements of the unitarity triangle angle γ using B ∓ → D K ∓ decays. The high CP -even content of D → π + π − π 0 , in particular, makes this a promising mode for improving the precision on γ . [ABSTRACT FROM AUTHOR]