  We present statistically robust observational evidence which imposes new
limits on dynamo saturation in young solar-type stars. These are inferred from
the increasing amplitude of the V-band optical flux with rotation, caused by
the filling of the disc with surface spots in a non-axisymmetric pattern.
Assuming spot coverage acts as a tracer of the total magnetic surface flux we
find that the magnetic activity saturates at a level at least 6 -- 10 times
greater than that inferred from chromospheric and transition line indicators.
We suggest that these new limits imply a minimum rotation for saturation of the
dynamo and that for high rotation rates starspot coverage acts as an
alternative diagnostic for the stellar dynamo to the chromospheric and
transition region line emission fluxes. The fact that the dynamo does not
appear to saturate at the low rotation rates indicated by chromospheric
indicators should assist evolutionary braking models that have to explain the
sudden spin-down of young fast rotating G-dwarfs. ( + 3 figures, available on
request) To appear in Astronomy and Astrophysics (A&A latex style file,
laa.sty, included at beginning of file)
