2026ApJ...998L...9S

The Wesenheit function is widely used to reduce the effects of interstellar reddening in distance measurements. Its construction, however, relies on the assumption of a universal extinction curve and on fixed values of the total-to-selective extinction ratio, R_V. Recent studies have shown that R_V varies significantly across the Milky Way and between different galaxies, raising concerns about systematic biases in Wesenheit magnitudes and period-Wesenheit relations. In this work, we discuss the impact of nonuniversal extinction on Wesenheit indices by combining the R_V-dependent extinction curve with a grid of stellar atmosphere models. We compute the integrated extinction in optical and near-infrared passbands, derive R_V-dependent R coefficients for multiple Wesenheit indices, and examine how changes in R_V propagate into Wesenheit magnitudes and Cepheid distances in our Galaxy. We find that the R coefficients in the Wesenheit functions vary strongly with R_V. For classical Cepheids in the Milky Way disk, variations of R_V within the typical observed range (2.6-3.6) can lead to substantial differences in the Wesenheit function, reaching ± 0.7 mag from the mean for the Gaia-based Wesenheit index W_G and resulting in distance errors of almost 40%. Near-infrared Wesenheit indices are much less sensitive to R_V changes. Our results clearly show that accounting for variable R_V is essential when applying period-Wesenheit relations, particularly in the optical regime, or that near- or mid-infrared-based distances should be used. While we present this effect for classical Cepheids, it applies to all pulsating stars for which period-Wesenheit relations are used to infer distances.