Dynamical constraints on the quadrupole mass moment of the HD 209458 star

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.