  We present for the first time phase-resolved UV spectroscopy of an Of?p star,
namely, HD 191612. The observations were acquired with the Space Telescope
Imaging Spectrograph (STIS) on-board the Hubble Space Telescope (HST). We
report the variability observed in the main photospheric and wind features and
compare the results with previous findings for the Of?p star HD 108. We show
that UV line strengths, H(alpha), and longitudinal magnetic field, vary
coherently according to the rotational period (P = 537.6d), providing
additional support for the magnetic oblique rotator scenario. The stellar and
wind parameters of HD 191612 are obtained based on NLTE expanding atmosphere
models. The peculiar wind line profile variations revealed by the new STIS data
- not reproduced by 1D atmosphere models - are addressed through non-spherical
MHD simulations coupled with radiative transfer. The basic aspects of the UV
variability observed are explained and the structure of the dynamical
magnetosphere of HD 191612 is discussed.
