Planet observations of microlensing event OGLE-1999-BUL-23: Limb-darkeningmeasurement of the source star

We present PLANET observations of OGLE-1999-BUL-23, a binary-lens microlens ing event toward the Galactic bulge. PLANET observations in the I and V ban ds cover the event from just before the first caustic crossing until the en d of the event. In particular, a densely sampled second caustic crossing en ables us to derive the linear limb-darkening coefficients of the source sta r : and c(V) = 0.786(-0.078)(+0.080) c(I) = 0.632(-0.037)(+0.047). 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 similar or e qual to 4800 K). The resulting linear limb-darkening coefficient of the sou rce is consistent with theoretical predictions, although it is likely that nonlinearity of the stellar surface brightness profile complicates the inte rpretation, especially for the I band. The global light curve fit to the da ta indicates that the event is due to a binary lens of a mass ratio q simil ar or equal to 0.39 and a projected separation d similar or equal to 2.42. The lens/source relative proper motion is (22.8 +/- 1.5) km s(-1) kpc(-1), typical of bulge/bulge or bulge/disk events.