good fit as the two spot model but with six fewer parameters. The transmission spectrum resulting from the facula model is also shown in Fig. \[fig:trans\_spec\] (red triangles). In contrast to the flat transmission spectrum resulting from the fitting of spots, the fitting of a facula led to a slope in the planetary radius increasing towards the red (Fig. \[fig:trans\_spec\]).

![image](wasp52_fit_facula.pdf)

[l c c c c ]{} Parameter & Symbol & $u'$ & $g'$ & NaI\
*2 spot model* & & & &\
Time of mid-transit (MJD)$^a$ & $T_0$ & — & 56178.12742 $ \pm 0.00005$ & —\
Scaled semi major axis$^a$ & $a/R_{*}$ & — & 6.99 $ \pm 0.04$ & —\
Impact parameter$^a$ & $b$ & — & 0.656 $^{+0.006}_{-0.007}$ & —\
Radius ratio & $R_{P}/R_{*}$ & 0.1759 $^{+0.0005}_{-0.0004}$ & 0.1751 $ \pm 0.0004$ & 0.1757 $ \pm 0.0008$\
Limb darkening coeff.$^b$ & $u_1$ & 0.643 & 0.529 & 0.381\
Limb darkening coeff. & $u_2$ & 0.169 $ \pm 0.008$ & 0.233 $ \pm 0.008$ & 0.290 $^{+0.009}_{-0.010}$\
Spot 1 x position$^a$ & $x_{1}$ & — & -0.19 $ \pm 0.03$ & —\
Spot 1 y position$^a$ & $y_{1}$ & — & 0.91 $^{+0.03}_{-0.04}$ & —\
Spot 1 