Millimagnitude photometry for transiting extrasolar planetary candidates.: III. accurate radius and period for OGLE-TR-111-B

We present accurate V-band photometry for a planetary transit of OGLE-TR-111 acquired with VIMOS at the ESO Very Large Telescope. The measurement of this transit allows us to refine the planetary radius, obtaining R-p 1.01 +/- 0.06 R-J. Given the mass of M-p 0.53 M-J previously measured from radial velocities, we confirm that the density is p(p) = 0.6 +/- 0.2 g cm(-3). We also revise the ephemeris for OGLE-TR-111-b, obtaining an accurate orbital period P = 4.014484 +/- 0.000014 days and predicting that the next observable transits would occur around December 2006, and after that only in mid-2008. Even though this period is different from previously published values, we cannot yet rule out a constant period.