  Rapidly rotating Neutron Stars in Low Mass X-ray Binaries (LMXBs) may be an
interesting source of Gravitational Waves (GWs). In particular, several modes
of stellar oscillation may be driven unstable by GW emission, and this can lead
to a detectable signal. Here we illustrate how current X-ray and ultra-violet
(UV) observations can constrain the physics of the r-mode instability. We show
that the core temperatures inferred from the data would place many systems well
inside the unstable region predicted by standard physical models. However, this
is at odds with theoretical expectations. We discuss different mechanisms that
could be at work in the stellar interior, and we show how they can modify the
instability window and make it consistent with the inferred temperatures.
