Observation of psi(3770) --> gamma chi_c0

From e+e- collision data acquired with the CLEO-c detector at CESR, we search for the non-DDbar decays psi(3770) -> gamma chi_cJ, with chi_cJ reconstructed in four exclusive decays modes containing charged pions and kaons. We report the first observation of such decays for J=0 with a branching ratio of (0.73+-0.07+-0.06)%. The rates for different J are consistent with the expectations assuming psi(3770) is predominantly a 1^3D_1 state of charmonium, but only if relativistic corrections are applied.