The strange close binary V405 And: the influence of different dynamos on the components

V405 And is a fast-rotating (P=0.465d) grazing eclipsing binary with two active components. Using multicolor photometric monitoring and radial velocity measurements, we find that the primary and the secondary components have masses of 0.49 and 0.21 solar masses, meaning that the primary probably possesses a radiative core and a convective envelope, while the secondary is fully convective. The radius of the low-mass component fits well the theoretical mass-radius relation. However, the radius of the primary is significantly larger than the predicted value, in fact, the discrepancy is far the highest from the four similar objects known.