  The binary system RW Aur consists of two classical T~Tauri stars (CTTSs). The
primary recently underwent its second observed major dimming event ($\Delta
V\,\sim2\,$mag). We present new, resolved Chandra X-ray and UKIRT near-IR (NIR)
data as well as unresolved optical photometry obtained in the dim state to
study the gas and dust content of the absorber causing the dimming. The X-ray
data show that the absorbing column density increased from
$N_H<0.1\times10^{22}\,$cm$^{-2}$ during the bright state to
$\approx2\times10^{22}\,$cm$^{-2}$ in the dim state. The brightness ratio
between dim and bright state at optical to NIR wavelengths shows only a
moderate wavelength dependence and the NIR color-color diagram suggests no
substantial reddening. Taken together, this indicates gray absorption by large
grains ($\gtrsim1\,\mu$m) with a dust mass column density of
$\gtrsim2\times10^{-4}\,$g$\,$cm$^{-2}$. Comparison with $N_H$ shows that an
absorber responsible for the optical/NIR dimming and the X-ray absorption is
compatible with the ISM's gas-to-dust ratio, i.e., that grains grow in the disk
surface layers without largely altering the gas-to-dust ratio. Lastly, we
discuss a scenario in which a common mechanism can explain the long-lasting
dimming in RW Aur and recently in AA Tau.
