  Context: Photometric observations for the OGLE-II microlens monitoring
campaign have been taken in the period 1997-2000. All light curves of this
campaign have recently been made public. Our analysis of these data has
revealed 13 low-amplitude transiting objects among ~15700 stars in three Carina
fields towards the galactic disk. One of these objects, OGLE2-TR-L9 (P~2.5
days), turned out to be an excellent transiting planet candidate.
  Aims: In this paper we report on our investigation of the true nature of
OGLE2-TR-L9, by re-observing the photometric transit with the aim to determine
the transit parameters at high precision, and by spectroscopic observations, to
estimate the properties of the host star, and to determine the mass of the
transiting object through radial velocity measurements.
  Methods: High precision photometric observations have been obtained in g',
r', i', and z' band simultaneously, using the new GROND detector, mounted on
the MPI/ESO 2.2m telescope at La Silla. Eight epochs of high-dispersion
spectroscopic observations were obtained using the fiber-fed FLAMES/UVES
Echelle spectrograph, mounted on ESO's Very Large Telescope at Paranal.
  Results: The photometric transit, now more than 7 years after the last
OGLE-II observations, was re-discovered only ~8 minutes from its predicted
time. The primary object is a fast rotating F3 star, with vsini=39.33+-0.38
km/s, T=6933+-58 K, log g = 4.25+-0.01, and [Fe/H] = -0.05+-0.20. The
transiting object is an extrasolar planet with M_p=4.5+-1.5 M_Jup and
R_p=1.61+-0.04 R_Jup. The rejection of possible blend scenarios was based on a
quantitative analysis of the multi-color photometric data [abridged].
