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GU Boo is one of only a relatively small number of well studied double-linedeclipsing binaries that contain low-mass stars. L'opez-Morales & Ribas (2005,hereafter LR05) present a comprehensive analysis of multi-color light andradial velocity curves for this system. The GU Boo light curves presented inLR05 had substantial asymmetries, which were attributed to large spots. Inspite of the asymmetry LR05 derived masses and radii accurate to about 2%. Weobtained additional photometry of GU Boo using both a CCD and a single-channelphotometer and modeled the light curves with the ELC software to determine ifthe large spots in the light curves give rise to systematic errors at the fewpercent level. We also modeled the original light curves from LR05 using modelswith and without spots. We derived a radius of the primary of 0.6329 +- 0.0026Solar-Radii, 0.6413 +- 0.0049 SR, and 0.6373 +- 0.0029 SR from the CCD,photoelectric, and LR05 data, respectively. Each of these measurements agreeswith the value reported by LR05 (R1=0.623 +- 0.016 SR) at the level of about2%. In addition, the spread in these values is about 1-2% from the mean. Forthe secondary, we derive radii of 0.6074 +- 0.0035 SR, 0.5944 +- 0.0069 SR, and0.5976 +- 0.0059 SR from the three respective data sets. The LR05 value isR2=0.620 +- 0.020 SR, which is about 2-3% larger than each of the three valueswe found. The spread in these values is about 2% from the mean. The systematicdifference between our three determinations of the secondary radius and that ofLR05 might be attributed to differences in the modelling process and codesused. Our own fits suggest that, for GU Boo at least, using accurate spotmodelling of a single set of multicolor light curves results in radiideterminations accurate at the 2% level.

The Effect of Star Spots on Accurate Radius Determination of the Low Mass Double-lined Eclipsing Binary GU Boo