  The dynamical evolution of tight star-planet systems is influenced by tidal
interactions between the star and the planet, as was shown recently. The rate
at which spins and orbits in such a system evolve depends on the stellar and
planetary tidal dissipation efficiency. Here, we present a method to constrain
the modified tidal quality factor $Q'_*$ of a planet-hosting star whose
rotational evolution has been altered by its planet through angular momentum
transfer from the planetary orbital motion to the rotation of the stellar
convective zone. The altered rotation is estimated from an observed discrepancy
of magnetic activity of the planet-hosting star and a coeval companion star,
i.e. this method is applicable to star-planet systems with wide stellar
companions. We give an example of the planet-hosting wide binary system
HD189733 and find that the planet host's modified tidal quality factor is
constrained to be $Q'_* \leq 2.33 \times 10^7$.
