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\begin{document} 
\title{O STARS IN TRANSITION. II. FUNDAMENTAL PROPERTIES\\
~\\
AND EVOLUTIONARY STATUS OF OFPE/WN9 STARS\\
~\\
FROM {\it HST\/} ULTRAVIOLET OBSERVATIONS\thanks{Based 
on observations with the NASA/ESA {\it Hubble Space Telescope}, 
obtained at the Space Telescope Science Institute, which is operated
by AURA for NASA under contract NAS5-26555}}

\author
{Anna Pasquali,\thanks{Space Telescope Science Institute, 3700 San 
Martin Drive, Baltimore, MD 21218; leitherer@stsci.edu, nota@stsci.edu, 
pasquali@stsci.edu} 
Norbert Langer,\thanks{Max-Planck-Institut f\"ur Astrophysik, 
Karl-Schwarzschild-Str.~1, D-85740 Garching b.\ M\"unchen, Germany;
ntl@ipp-garching.mpg.de} 
Werner Schmutz,\thanks{Institut f\"ur Astronomie, ETH-Zentrum, CH-8092 
Z\"urich, Switzerland; schmutz@ astro.phys.ethz.ch}
Claus Leitherer,\samethanks{2}\\
\and
Antonella~Nota,\samethanks{2}\morethanks{Affiliated with the Astrophysics
Division, Space Science Department of the European Space Agency} 
Ivan Hubeny,\thanks{NASA Goddard Space Flight Center, Code 681, Greenbelt, 
MD 20771; hubeny@ achamp.gsfc.nasa.gov} 
Anthony F.~J.\ Moffat\/\thanks{D\'epartement de Physique, Universit\'e de 
Montreal, C.P.~6128, Succ.\ Centreville, Montr\'eal, Qc, Canada H3C3J7; 
moffat@astro.umontreal.ca}\morethanks{Observatoire du Mont M\'egantic}
}

\pub {The Astrophysical Journal}
\recacc{26 April 1996}{12 September 1996}
\maketitle

\abstract{We present new {\it HST/FOS\/} ultraviolet spectroscopic 
observations of seven LMC Ofpe/WN9 stars.
We find that Ofpe/WN9 stars have slow winds
with terminal velocities of about 400~km~s$^{-1}$ and high 
mass-loss rates of the order of 2--$5\times10^{-5}$~\Myr. Ofpe/WN9 stellar
temperatures and radii are in the range 30,000--39,000~K, and 19--39~\Rs,
respectively. Stellar luminosities are between log(L/L$_{\odot}$) = 5.6
and 6.3.

We study the Ofpe/WN9 stars winds and examine their evolutionary status. We 
find that Ofpe/WN9 stars are intermediate between O and WR~stars in terms of 
the wind momentum flux. We also find that the stellar properties and wind 
momentum of the Ofpe/WN9 sample place them in the evolutionary sequence:
O $\ra$ Of $\ra$ H-rich WNL $\ra$ Ofpe/WN9, for initial stellar masses
less than $\sim100$~\Ms. 

In view of persisting discrepancies of standard massive star models with
observations, we compute massive main sequence models according to three
different evolutionary scenarios. We find that both, higher mass-loss rate 
and enhanced mixing
between core and envelope, are required in order to yield models compatible
with the derived stellar and wind properties of Ofpe/WN9 stars. The emerging
picture may be consistent with earlier evidence of Ofpe/WN9 stars being 
quiescent LBVs. This idea is further strengthened by the highly reduced
surface H~mass fractions of the Ofpe/WN9 stars. We derive X$_s$ = 0.5 to 0.3,
which still excludes Ofpe/WN9 stars from being core He burning object, but
is almost identical to the X$_s$ values recently measured in LBVs.}