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We present PLANET observations of OGLE-1999-BUL-23, a binary-lensmicrolensing event towards the Galactic bulge. PLANET observations in the I andV bands cover the event from just before the first caustic crossing until theend of the event. In particular, a densely-sampled second caustic crossingenables 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}. Combinedanalysis of the light curve and the color-magnitude diagram suggests that thesource star is a G/K subgiant in the Galactic bulge (T_eff = 4800 K). Theresulting linear limb-darkening coefficient of the source is consistent withtheoretical predictions, although it is likely that non-linearity of thestellar surface brightness profile complicates the interpretation, especiallyfor the I band. The global light curve fit to the data indicates that the eventis 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, typicalof bulge/bulge or bulge/disk events.

PLANET Observations of Microlensing Event OGLE‐1999‐BUL‐23: Limb‐darkening Measurement of the Source Star