  The aim of this paper is to dynamically constrain the quadrupole mass moment
J_2 of the main-sequence HD 209458 star. The adopted method is the
confrontation between the measured orbital period of its transiting planet
Osiris and a model of it. Osiris is assumed to move along an equatorial and
circular orbit. Our estimate, for given values of the stellar mass M and radius
R and by assuming the validity of general relativity, is J_2=(3.5 +/- 385.1)
10^-5. Previous fiducial evaluations based on indirect, spectroscopic
measurements yielded J_2\sim 10^-6: such a value is compatible with our result.
