  We present the results of a detailed spectroscopic analysis of 20 years of
observations of AG Carinae using the radiative transfer code CMFGEN. Among the
conclusions of this work, we highlight the importance of including
time-dependent effects in the analysis of the full S Dor cycle. We obtained
that the mass-loss rate is approximately constant during the cool phases,
implying that the S Dor-type eruptions begin well earlier than the maximum seen
in the visual lightcurve. We also determined that the S Dor cycles are
ultimately a consequence of an increase/decrease of the hydrostatic radius in
combination with the formation of a pseudo-photosphere.
