We report a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO_2 bilayers, show a non-uniform increase below ~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering peaks as a manifestation of a charge density wave in the CuO_2 planes coupled to the oxygen-vacancy ordering.
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