  Neutron stars spin down over time due to a number of energy-loss processes.
We provide tantalizing population-based evidence that millisecond pulsars
(MSPs) have a minimum ellipticity of $\epsilon\approx10^{-9}$ around their spin
axis and that, consequently, some spin down mostly through gravitational-wave
emission. We discuss the implications of such a minimum ellipticity in terms of
the internal magnetic field strengths and nuclear matter composition of neutron
stars and show it would result in the Advanced LIGO and Virgo
gravitational-wave detectors, or their upgrades, detecting gravitational waves
from some known MSPs in the near future.
