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ZZ UMa is a detached eclipsing binary with an orbital period of 2.299 d thatshows total eclipses and starspot activity. We used five sectors of lightcurves from the Transiting Exoplanet Survey Satellite (TESS) and two publishedsets of radial velocities to establish the properties of the system to highprecision. The primary star has a mass of 1.135 +/- 0.009 Msun and a radius of1.437 +/- 0.007 Rsun, whilst the secondary component has a mass of 0.965 +/-0.005 Msun and a radius of 1.075 +/- 0.005 Rsun. The properties of the primarystar agree with theoretical predictions for a slightly super-solar metallicityand an age of 5.5 Gyr. The properties of the secondary star disagree with theseand all other model predictions: whilst the luminosity is in good agreementwith models the radius is too large and the temperature is too low. These arethe defining characteristics of the radius discrepancy which has been known for40 years but remains an active area of research. Starspot activity is evidentin the out-of-eclipse portions of the light curve, in systematic changes in theeclipse depths, and in emission at the Ca H and K lines in a medium-resolutionspectrum of the system. Over the course of the TESS observations the light andsurface brightness ratios between the stars change linearly by 20% and 14%,respectively, but the geometric parameters do not. Studies of objects showingspot activity should account for this by using observations over long timeperiods where possible, and by concentrating on totally-eclipsing systems whoselight curves allow more robust measurements of the physical properties of thesystem.

Rediscussion of eclipsing binaries. Paper XI. ZZ Ursae Majoris, a solar-type system showing total eclipses and a radius discrepancy