  V405 And is an ultrafast-rotating (P_rot ~ 0.46 days) eclipsing binary. The
system consists of a primary star with radiative core and convective envelope,
and a fully convective secondary. Theories have shown, that stellar structure
can depend on magnetic activity, i.e., magnetically active M-dwarfs should have
larger radii. Earlier light curve modelling of V405 And indeed showed this
behaviour: we found that the radius of the primary is significantly larger than
the theoretically predicted value for inactive main sequence stars (the
discrepancy is the largest of all known objects), while the secondary fits well
to the mass-radius relation. By modelling our recently obtained light curves,
which show significant changes of the spotted surface of the primary, we can
find further proof for this phenomenon.
