  As is well known, gravitational wave detections of coalescing binaries are
standard sirens, allowing a measurement of source distance by gravitational
wave means alone. In this paper we explore the analogue of this capability for
continuous gravitational wave emission from individual spinning neutron stars,
whose spin-down is driven purely by gravitational wave emission. We show that
in this case, the distance measurement is always degenerate with one other
parameter, which can be taken to be the moment of inertia of the star. We
quantify the accuracy to which such degenerate measurements can be made. We
also discuss the practical application of this method to scenarios where one or
other of distance or moment of inertia is constrained, breaking this degeneracy
and allowing a measurement of the remaining parameter. We consider a broad
range of possible, unknown parameters, as well as we present results for the
aLIGO and Einstein Telescope sensitivities. Our results will be of use
following the eventual detection of a neutron star spinning down through such
gravitational wave emission.
