HD 209458 planetary transits from Hipparcos photometry

During its 39-month mission, Hipparcos observed the star HD 209458 on 89 oc casions. Five of these observations are shown to correspond to epochs of pl anetary transits, with a 2.3 +/- 0.4% mean decrease of flux in the Hp band being observed during the transits. As a result of the long temporal baseli ne of more than 8 years between the Hipparcos and radial velocity measureme nts, an accurate orbital period of P = 3.524739 +/- 0.000014 days can be de rived from the Hipparcos photometric data.