  In this paper we use the 7 mm and 1.3 mm light curves obtained during the
2003.5 low excitation phase of the eta Carinae system to constrain the possible
parameters of the binary orbit. To do that we assumed that the mm wave emission
is produced in a dense disk surrounding the binary system; during the low
excitation phase, which occurs close to periastron, the number of ionizing
photons decreases, producing the dip in the radio emission. On the other hand,
due to the large eccentricity, the density of the shock region at periastron is
very high and the plasma is optically thick for free-free radiation at 7 mm,
explaining the sharp peak that was observed at this frequency and lasted for
about 10 days. From the shape and duration of the peak we were able to
determine the orbital parameters of the binary system, independently of the
stellar parameters, such as mass loss rates, wind velocities or temperature at
the post-shock region.
