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tokens: For example, at the 0.75 bandwidth, the average depth of Night 1 and Night 3 in W2015 is 0.40% while it is 0.512% in our work. Barnes et al. (2016) also calculated the Na i D lines’ absorption depth and their result is 0.45% (average of Night 1 and Night 3) which is also larger than in W2015 but smaller than ours. The differences are probably due to the data reduction method, however, these differences are not significant considering the measurement uncertainties. When fitting the Na i absorption, we used light curve models which include the limb-darkening within the bandwidths while the other two papers use a box light curve model. As for the CLV effect, there is no obvious CLV feature in the light curve in W2015 (Fig. 3 of their paper), while the light curve in Barnes et al. ( 2016 , Fig. 6 of their paper) shows a similar CLV feature to that in our work. In general, the light curves in Barnes et al. (2016) and our work agree more closely than with W2015. Fig. 4 Observed light curves for the Mg i line (red lines). The black line is the CLV model. 4. Comparison of the CLV effect for HD 189733b and HD 209458b The actual CLV effect varies between different exoplanet systems. In order to illustrate and study the different manifestations of the CLV effect, we present the CLV of the HD 209458b system and compare it with the HD 189733b system. These two transiting exoplanet systems have bright host stars and have been extensively studied, thus they serve as typical samples for the study the CLV effect. We modelled the CLV effect of HD 209458b using the parameters shown in Table 2 . In order to compare the strengths of the CLV effect with the Na i absorption, we adopted the measured planetary absorption values from ( Snellen et al. 2008 , S2008). The results are shown in Fig. 5 and Table 4 . We note that the calculation in S2008 does not correct for the CLV effect. However, since the overall CLV contribution is relatively small (the value is at the noise level of their observation data), adopting the S2008’s value as the Na i absorption is justified in our study. Table 2 Parameters of the HD 209458b system. Notes. The transit epoch and the period are from Knutson et al. (2007) and all the other values are from Torres et al. (2008) . Fig. 5 Comparison of the CLV effect between HD 189733b and HD 209458b. Here the calculations are for the Na i D lines (average of and ). The first row is for HD 189733b and the second row is for HD 209458b. The three columns are for the bandwidths of 0.75, 1.5 and 3.0. The green lines are the modelled CLV light curves while the red lines are the Na i absorption light curves, and the black lines are the combination of the two. We note that the y axis scales are different for the two rows: the values for HD 189733b are significantly larger than for HD 209458b. In order to compare the two systems, we plot their CLV models together with the Na absorption at three different bandwidths in Fig. 5 . There are several