  Inaccurate limb-darkening models can be a significant source of error in the
analysis of the light curves for transiting exoplanet and eclipsing binary star
systems. To test the accuracy of published limb-darkening models, I have
compared limb-darkening profiles predicted by stellar atmosphere models to the
limb-darkening profiles measured from high-quality light curves of 43 FGK-type
stars in transiting exoplanet systems observed by the Kepler and TESS missions.
The comparison is done using the parameters $h^{\prime}_1 =
I_{\lambda}(\frac{2}{3})$ and $h^{\prime}_2 = h^{\prime}_1 -
I_{\lambda}(\frac{1}{3})$, where $I_{\lambda}(\mu)$ is the specific intensity
emitted in the direction $\mu$, the cosine of the angle between the line of
sight and the surface normal vector. These parameters are straightforward to
interpret and insensitive to the details of how they are computed. I find that
most (but not all) tabulations of limb-darkening data agree well with the
observed values of $h^{\prime}_1$ and $h^{\prime}_2$. There is a small but
significant offset $\Delta h^{\prime}_1 \approx 0.006$ compared to the observed
values that can be ascribed to the effect of a mean vertical magnetic field
strength $\approx 100$\,G that is expected in the photospheres of these
inactive solar-type stars but that is not accounted for by typical stellar
model atmospheres. The implications of these results for the precision of
planetary radii measured by the PLATO mission are discussed briefly.
