  GU Boo is one of only a relatively small number of well studied double-lined
eclipsing binaries that contain low-mass stars. L'opez-Morales & Ribas (2005,
hereafter LR05) present a comprehensive analysis of multi-color light and
radial velocity curves for this system. The GU Boo light curves presented in
LR05 had substantial asymmetries, which were attributed to large spots. In
spite of the asymmetry LR05 derived masses and radii accurate to about 2%. We
obtained additional photometry of GU Boo using both a CCD and a single-channel
photometer and modeled the light curves with the ELC software to determine if
the large spots in the light curves give rise to systematic errors at the few
percent level. We also modeled the original light curves from LR05 using models
with and without spots. We derived a radius of the primary of 0.6329 +- 0.0026
Solar-Radii, 0.6413 +- 0.0049 SR, and 0.6373 +- 0.0029 SR from the CCD,
photoelectric, and LR05 data, respectively. Each of these measurements agrees
with the value reported by LR05 (R1=0.623 +- 0.016 SR) at the level of about
2%. In addition, the spread in these values is about 1-2% from the mean. For
the secondary, we derive radii of 0.6074 +- 0.0035 SR, 0.5944 +- 0.0069 SR, and
0.5976 +- 0.0059 SR from the three respective data sets. The LR05 value is
R2=0.620 +- 0.020 SR, which is about 2-3% larger than each of the three values
we found. The spread in these values is about 2% from the mean. The systematic
difference between our three determinations of the secondary radius and that of
LR05 might be attributed to differences in the modelling process and codes
used. Our own fits suggest that, for GU Boo at least, using accurate spot
modelling of a single set of multicolor light curves results in radii
determinations accurate at the 2% level.
