  We report the first detection of a planetary transit by spectroscopic
measurements. We have detected the distortion of the stellar line profiles
during a planetary transit. With the ELODIE spectrograph we took a sequence of
high precision radial velocities of the star HD209458 at time of a transit of
its planet. We detected an anomaly in the residuals of the orbit. The shape and
the amplitude of the anomaly are modeled as a change of the mean stellar line
profile resulting from the planet crossing the disk of the rotating star. The
planetary orbit is in the same direction as the stellar rotation. Using the
photometric transit to constrain the timing and the impact parameters of the
transit, we measure an angle alpha=3.9d between the orbital plane and the
apparent equatorial plane as well as a vsini=3.75(+-)1.25 kms-1. With
additional constrains on the inclination of the star and on the statistics of
the line of sight distribution, we can set an upper limit of 30d to the angle
between the orbital plane and the stellar equatorial plane.
