  In this talk I review how a non-zero cosmological constant $\Lambda$ affects
the propagation of gravitational waves and their detection in pulsar timing
arrays (PTA). If $\Lambda\neq 0$ it turns out that waves are anharmonic in
cosmological Friedmann-Robertson- Walker coordinates and although the amount of
anharmonicity is very small it leads to potentially measurable effects. The
timing residuals induced by gravitational waves in PTA would show a peculiar
angular dependence with a marked enhancement around a particular value of the
angle subtended by the source and the pulsars. This angle depends mainly on the
actual value of the cosmological constant and the distance to the source.
Preliminary estimates indicate that the enhancement can be rather notorious for
supermassive black hole mergers and in fact it could facilitate the first
direct detection of gravitational waves while at the same time representing a
`local' measurement of $\Lambda$.
