also supported by the Polish Ministry of Science and Higher Education (MNiSW) through the program “Iuventus Plus” award No. IP2011 026771. AG acknowledges supported by NSF grant AST 1103471.

Microlensing model parameters from analytical approximation
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The idea that the observed characteristics of the planetary microlensing event give estimates of lens parameters was presented long time ago [@gould92; @gaudi97]. Here we present an example of such calculations, which yield basic point-lens ($u_0$, $t_{\rm E}$) and static binary-lens parameters ($\alpha_0$, $s_0$, $q$). The rectilinear motion of the source and absence of blending are assumed. The geometry of the source trajectory compared to the planetary caustic and the lens is presented in Figure \[fig:geo\].

The brightness difference between the baseline and the time when the source is closest to the lens is $0.78~{\rm mag}$. Thus, the maximum magnification $A_{\rm max} = 2.05$. It is related to the minimum impact parameter $u_0$ via @paczynski86 formula: $$A_{\rm max} = \frac{u^2_0+2}{u_0\sqrt{u^2_0+4}}$$ Inverting this relation yields $u_0 = 0.539$. The above equation can be used to estimate the magnification, and thus the brightness, of the event at epochs $t_0-t_{\rm E}$ and $t_0+t_{\rm E}$ where $t_0 = 6143.4$. The result is $16.09~{\rm mag}$. We can find the 