  Context: Several studies have so far placed useful constraints on planetary
atmospheric properties using transmission spectrsocopy, and in the case of
HD209458b even the radial velocity of the planet during the transit event has
been reconstructed opening a new range of possibilities. AIMS. In this
contribution we highlight the importance to account for the orbital
eccentricity and longitude of periastron of the planetary orbit to accurately
interpret the measured planetary radial velocity during the transit. Methods:
We calculate the radial velocity of a transiting planet in an eccentric orbit.
Given the larger orbital speed of planets with respect to their stellar
companions even small eccentricities can result in detectable blue or redshift
radial velocity offsets during the transit with respect to the systemic
velocity, the exact value depending also on the longitude of the periastron of
the planetary orbit. For an hot-jupiter planet, an eccentricity of only e=0.01
can produce a radial velocity offset of the order of the km/s. Conclusions: We
propose an alternative interpretation of the recently claimed radial velocity
blueshift (~2 km/s) of the planetary spectral lines of HD209458b which implies
that the orbit of this system is not exactly circular. In this case, the
longitude of the periastron of the stellar orbit is most likely confined in the
first quadrant (and that one of the planet in the third quadrant). We highlight
that transmission spectroscopy allows not only to study the compositional
properties of planetary atmospheres, but also to refine their orbital
parameters and that any conclusion regarding the presence of windflows on
planetary surfaces coming from transmission spectroscopy measurements requires
precise known orbital parameters from RV.
