Fig. 1

Observed spectrum of HD 189733. The top panel shows the three bandwidths (0.75Å, 1.5Å and 3.0Å) used to measure the relative flux of the Na i D lines. These passbands are centred at the line cores of D1 and D2. The reference passbands at the blue and red parts of the Na i D lines are also shown. The bottom panel is the zoom at the the Na i D lines.
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