The Ofpe/WN9 Stars in M33

We present Hubble Space Telescope /STIS Ultraviolet spectra of the six known Ofpe/WN9 stars ("slash stars") in M33. These stars were selected for showing the characteristics of the Ofpe/WN9 class from previous optical ground-based spectroscopy. The UV spectra are rich in wind lines whose strength and terminal velocity vary greatly across our target sample. We analyze the STIS spectra with non-LTE, line-blanketed, spherical models with hydrodynamics, computed with the WM-BASIC code. We find C to be underabundant and N overabundant with respect to the solar values, with a ratio (by mass) of C/N between 0.02 and 0.9 across the sample. Some stars show very conspicuous wind lines (P Cygni profiles), while two stars have extremely weak winds. The mass-loss rates thus vary greatly across the sample. The mass-loss rates of the hottest stars are lower than typical values of WNL stars but higher than expected for normal Population I massive stars. There is indication that the mass-loss rates may be variable in time. The C/N ratio and the other physical parameters derived by the spectral modeling ( T eff , L bol , mass) are consistent with evolutionary calculations for objects with moderately high initial masses (≈30-50 M ☉ ), evolving toward the WNL stage through an enhanced mass-loss phase.