Probing the Missing Link between the Diffuse Interstellar Bands and the
  Total-to-Selective Extinction Ratio $R_V$ $-$ I. Extinction versus Reddening
  The carriers of the still (mostly) unidentified diffuse interstellar bands
(DIBs) have been a long-standing mystery ever since their first discovery
exactly 100 years ago. In recent years, the ubiquitous detection of a large
number of DIBs in a wide range of Galactic and extragalactic environments has
led to renewed interest in connecting the occurrence and properties of DIBs to
the physical and chemical conditions of the interstellar clouds, with
particular attention paid to whether the DIB strength is related to the shape
of the interstellar extinction curve. To shed light on the nature and origin of
the DIB carriers, we investigate the relation between the DIB strength and
$R_V$, the total-to-selective extinction ratio, which characterizes how the
extinction varies with wavelength (i.e., the shape of the extinction curve). We
find that the DIB strength and $R_V$ are not related if we represent the
strength of a DIB by its reddening-normalized equivalent width (EW), in
contrast to the earlier finding of an anticorrelation in which the DIB strength
is measured by the extinction-normalized EW. This raises a fundamental question
about the appropriate normalization for the DIB EW.
