**Table 5:** The $N_{\rm H}$ /A_V ratios from the literature (see also Table 1).
$N_{\rm H}$ /A_V	Ref.
$1.8 \times10^{21}$ cm^-2 ^a	Bohlin et al. (1978)
$1.9 \times10^{21}$ cm^-2 ^a	Whittet (1981)
$2.2 \times10^{21}$ cm^-2 ^a,b	Gorenstein (1975); Ryter (1996)
$2.0 \times10^{21}$ cm^-2 ^b	Ryter et al. (1975)
$1.8 \times10^{21}$ cm^-2 ^b	Predehl & Schmitt (1995)

^a UV; ^b X-rays.

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