Past studies based on optical spectroscopy suggest that the five ω Cen
pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars
with effective temperatures of around 50 000 K. This places the stars below the
red edge of the theoretical instability strip in the log g− Teff diagram,
where no pulsation modes are predicted to be excited. Our goal is to determine
whether this temperature discrepancy is real, or whether the stars' effective
temperatures were simply underestimated. We present a spectral analysis of two
rapidly pulsating extreme horizontal branch (EHB) stars found in ω Cen.
We obtained Hubble Space Telescope/COS UV spectra of two ω Cen
pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines
to better constrain their effective temperatures. As a by-product we also
obtained FUV lightcurves of the two pulsators. Using the relative strength of
the N IV and N V lines as a temperature indicator yields Teff values close to
60 000 K, significantly hotter than the temperatures previously derived. From
the FUV light curves we were able to confirm the main pulsation periods known
from optical data. With the UV spectra indicating higher effective temperatures
than previously assumed, the sdO stars would now be found within the predicted
instability strip. Such higher temperatures also provide consistent
spectroscopic masses for both the cool and hot EHB stars of our previously
studied sample.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic