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Limb-darkening is fundamental in determining transit lightcurve shapes, andis typically modeled by a variety of laws that parametrize the intensityprofile of the star that is being transited. Confronted with a transitlightcurve, some authors fix the parameters of these laws, the so-calledlimb-darkening coefficients (LDCs), while others prefer to let them float inthe lightcurve fitting procedure. Which of these is the best strategy, however,is still unclear, as well as how and by how much each of these can bias theretrieved transit parameters. In this work we attempt to clarify those pointsby first re-calculating these LDCs, comparing them to measured values fromKepler transit lightcurves using an algorithm that takes into accountuncertainties in both the geometry of the transit and the parameters of thestellar host. We show there are significant departures from predicted modelvalues, suggesting that our understanding of limb-darkening still needs toimprove. Then, we show through simulations that if one uses the quadraticlimb-darkening law to parametrize limb-darkening, fixing and fitting the LDCscan lead to significant biases -up to $\sim 3\%$ and $\sim 1\%$ in $R_p/R_*$,respectively-, which are important for several confirmed and candidateexoplanets. We conclude that, in this case, the best approach is to let theLDCs be free in the fitting procedure. Strategies to avoid biases in data frompresent and future missions involving high precision measurements of transitparameters are described.

Limb darkening and exoplanets: testing stellar model atmospheres and identifying biases in transit parameters