  We apply a model of dynamo-driven mass loss, magnetic braking and tidal
friction to the evolution of stars with cool convective envelopes; in
particular we apply it to binary stars where the combination of magnetic
braking and tidal friction can cause angular-momentum loss from the {\it
orbit}. For the present we consider the simplification that only one component
of a binary is subject to these non-conservative effects, but we emphasise the
need in some circumstances to permit such effects in {\it both} components.
  The model is applied to examples of (i) the Sun, (ii) BY Dra binaries, (iii)
Am binaries, (iv) RS CVn binaries, (v) Algols, (vi) post-Algols. A number of
problems regarding some of these systems appear to find a natural explanation
in our model. There are indications from other systems that some coefficients
in our model may vary by a factor of 2 or so from system to system; this may be
a result of the chaotic nature of dynamo activity.
