Resumen
We undertook a project aimed at the observational determination of the best fitting linb darkening law for contact binaries. Our sample consisted of systems exhibiting total eclipses, observed by the Kepler spacecraft. We focused our study on three most commonly used limb darkening laws: linear, logarythmic and square root.In the first part of this work, we investigate how the long cadence mode in the Kepler mission (resolution of about 30 minutes) influences the shape of light curves of eclipsing binaries. As an example we used simulated light curves of contact binaries with periods between 0.2 and 1.6 days, exhibiting flat bottom secondary minima. We found that the binning causes a decrease of amplitude of geometrical variations and change of the shape of minima. We modeled the simulated light curves with a code that does not account for binning. By comparing the derived parameters with the input ones, it turned out that only when a binary period is longer than about 1.5 days, the solutions derived with a code that does not account for binning, would be accurate.
We selected a sample of contact binaries observed by Kepler, exhibiting a flat bottom secondary minimum and no intrinsic activity. With the above conclusion in mind, we solved the light curves of selected systems with the most recent version of the Wilson-Devinney code, which accounts for binning and incorporates the limb darkening coefficients for linear, logarithmic and square root distributions, tabulated by Van Hamme. We derived the systems parameters and compared the solutions obtained for the three limb darkening laws. For nine systems, the best fit was derived for the linear limb darkening distribution, while the square root law for seven systems, and for just one, the logarithmic low was preferred.