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 \sim 15700 stars in three Carina fields towards the galactic disk .
One of these objects , OGLE2-TR-L9 ( P \sim 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 ^ { \prime } , r ^ { \prime } , i ^ { \prime } , and z ^ { \prime } 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 \sim 8 minutes from its predicted time .
The primary object is a fast rotating F3 star , with v sin i =39.33 \pm 0.38 km/s , T=6933 \pm 58 K , log g = 4.25 \pm 0.01 , and [ Fe/H ] = - 0.05 \pm 0.20 .
The transiting object is an extrasolar planet with M _ { p } =4.5 \pm 1.5 M _ { Jup } and R _ { p } =1.61 \pm 0.04R _ { Jup } .
Since this is the first planet found to orbit a fast rotating star , the uncertainties in the radial velocity measurements and in the planetary mass are larger than for most other planets discovered to date .
The rejection of possible blend scenarios was based on a quantitative analysis of the multi-color photometric data .
A stellar blend scenario of an early F star with a faint eclipsing binary system is excluded , due to the combination of 1 ) the consistency between the spectroscopic parameters of the star and the mean density of the transited object as determined from the photometry , and 2 ) the excellent agreement between the transit signal as observed at four different wavelengths .

Conclusions :