Top: DCO+ map from the disk of HD 163296 (Mathews et al. 2013) superimposed on our PDI K_S-band image. The white contours indicate the emission at 3σ and 4σ level (σ = 18 mJy beam^-1 km s^-1). The DCO+ emission is depleted inside ~0.5″, while it reaches its maximum roughly at the location of the dusty ring. Bottom: DCO+ intensity along the major axis vs. contrast of the PDI images calculated from Eq. (4).