  The members of the scarce category of Of^+ supergiants present properties
that are intermediate between regular O-stars and Wolf-Rayet (WR) stars.
Significant similarities between these transitional stars and WN-type objects
are now clearly established, at least in the visible and near-infrared domains,
pointing to common stellar wind properties. In this study, we report on the
first dedicated X-ray observations of HD16691 (O4If^+) and HD14947 (O5f^+),
revealing a soft thermal spectrum in agreement with the expected X-ray emission
from a single O-type star. However, the X-ray luminosity of our targets is
slightly lower than expected for single O-type stars, suggesting that the
particular properties of their stellar wind has also a significant impact on
the X-ray emission of these objects on the way to the WN category. We argue
that the X-ray under-luminosity of HD16691 and HD14947 may be interpreted as
the signature in X-rays of the intermediate stage between O and WR stars, as a
consequence of enhanced wind density.
