  We present PLANET observations of OGLE-1999-BUL-23, a binary-lens
microlensing event towards the Galactic bulge. PLANET observations in the I and
V bands cover the event from just before the first caustic crossing until the
end of the event. In particular, a densely-sampled second caustic crossing
enables us to derive the linear limb-darkening coefficients of the source star;
c_V = 0.786^{+0.080}_{-0.078} and c_I = 0.632^{+0.047}_{-0.037}. Combined
analysis of the light curve and the color-magnitude diagram suggests that the
source star is a G/K subgiant in the Galactic bulge (T_eff = 4800 K). The
resulting linear limb-darkening coefficient of the source is consistent with
theoretical predictions, although it is likely that non-linearity of the
stellar surface brightness profile complicates the interpretation, especially
for the I band. The global light curve fit to the data indicates that the event
is due to a binary lens of a mass ratio q = 0.39 and a projected separation d =
2.42. The lens/source relative proper motion is (22.8 +_ 1.5) km/s/kpc, typical
of bulge/bulge or bulge/disk events.
