  With a handful of measurements of limb-darkening coefficients, galactic
microlensing has already proven to be a powerful technique for studying
atmospheres of distant stars. Survey campaigns such as OGLE-III are capable of
providing ~ 10 suitable target stars per year that undergo microlensing events
involving passages over the caustic created by a binary lens which last from a
few hours to a few days and allow to resolve the stellar atmosphere by frequent
broadband photometry. For a caustic exit lasting 12 h and a photometric
precision of 1.5%, a moderate sampling interval of 30 min (corresponding to ~
25-30 data points) is sufficient for providing a reliable measurement of the
linear limb-darkening coefficient Gamma with an uncertainty of ~ 8% which
reduces to ~ 3% for a reduced sampling interval of 6 min for the surroundings
of the end of the caustic exit. While some additional points over the remaining
parts of the lightcurve are highly valuable, a denser sampling in these regions
provides little improvements. Unless a high-precision measurement is desired,
limb-darkening coefficients for several filters can be obtained or observing
time can be spent on other targets during the same night. The adoption of an
inappropriate stellar intensity profile yields characteristic systematics in
the model residuals which can be distinguished from those arising from effects
causing an acceleration between source and caustic which are however unlikely
to affect the lightcurve significantly for most events, although a free
acceleration parameter blurs the limb-darkening measurement if the passage
duration cannot be accurately determined.
