  Limb darkening (LD) is typically parameterised using a range of functional
"laws" in models of the light curves of eclipsing binary and transiting
planetary systems. The two-coefficient LD laws all suffer from a strong
correlation between their coefficients, preventing a reliable determination of
both coefficients from high-quality light curves. We use numerical simulations
to propose re-parameterisations of the quadratic, logarithmic, square-root and
cubic LD laws that show much weaker correlations, and implement them into the
JKTEBOP code. We recommend that these re-parameterisations are used whenever
both LD coefficients are fitted. Conversely, when fitting for only one
coefficient, the standard laws should be used to avoid problems with fixing
coefficients at poor values. We find that these choices have little effect on
the other fitted parameters of a light curve model. We also recommend that the
power-2 LD law should be used as default because it provides a good fit to
theoretical predictions, and that the quadratic and linear laws should be
avoided because they do not.
