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Diffuse interstellar bands (DIBs) are broad absorption features measured insightlines probing the diffuse interstellar medium. Although largecarbon-bearing molecules have been proposed as the carriers producing DIBs,their identity remains unknown. The sight line to the young massivestar-forming region M17 shows anomalous extinction in the sense that thetotal-to-selective extinction parameter differs significantly from the averageGalactic value and may reach values $R_{V} > 4$. Given the high $R_V$ values,we investigate whether the DIBs in sight lines towards young OB stars in M17show a peculiar behaviour. We measure the properties of the most prominent DIBsin M17 and study these as a function of $E(B-V)$ and $R_{V}$. The DIB strengthsin M17 concur with the observed relations between DIB equivalent width andreddening $E(B-V)$ in Galactic sight lines. For several DIBs we discover alinear relation between the normalised DIB strength EW/$A_{V}$ and$R_{V}^{-1}$. These trends suggest two groups: (i) a group of ten moderatelystrong DIBs that show a sensitivity to changes in $R_{V}$ that is modest andproportional to DIB strength, and (ii) a group of four very strong DIBs thatreact sensitively and to a similar degree to changes in $R_{V}$, but in a waythat does not appear to depend on DIB strength. The DIB behaviour as a functionof reddening is not peculiar in sight lines to M17. Also, we do not detectanomalous DIB profiles as seen in Her 36. DIBs are stronger, per unit visualextinction, in sight lines characterised by a smaller value of $R_{V}$ (largefraction of small dust particles). New relations between extinction normalisedDIB strengths, EW/$A_V$, and $R_V$ support the idea that DIB carriers andinterstellar dust are connected. Given the distinct behaviour of two groups ofDIBs, different types of carriers do not necessarily relate to the dust grainsin a similar way.

Diffuse interstellar bands in the HII region M17: Insights into their relation with the total-to-selective visual extinction $R_V$