  In this article we show that the, mainly spacetime, w-modes may become
unstable in a rotating ultracompact ($R<3M$) relativistic star. We provide
results for the axial modes of a rotating star in the slow rotation
approximation, and demonstrate that an instability may become active in stars
with an ergosphere. These results suggest that the so-called ``ergoregion
instability'' discussed by other authors is, in fact, associated with the
w-modes of the star. The unstable modes are found to grow on a timescale of
seconds to minutes. We discuss briefly whether the instability is likely to
have any astrophysical relevance.
