We report a study of the suppressed decays B - → [K +π -] DK - and B - → [K +π -] Dπ -, where [K +π -] D indicates that the K +π - pair originates from a neutral D meson. These decay modes are sensitive to the unitarity triangle angle φ 3. We use a data sample containing 275 × 10 6 BB̄ pairs recorded at the Y(4S) resonance with the Belle detector at the KEKB asymmetric e +e - storage ring. The signal for B - → [K +π -] DK - is not statistically significant, and we set a limit r B < 0.27 at 90% confidence level, where r B is the magnitude of the ratio of amplitudes |A(B - → D̄ 0K -)/A(B - → D 0K -)|. We observe a signal with 6.4σ statistical significance in the related mode, B - → [K +π -] Dπ -.
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