  The link between the electroweak gauge boson masses and the Fermi constant
via the muon lifetime measurement is instrumental for constraining and
eventually pinning down new physics. We consider the simplest extension of the
Standard Model with an additional real scalar SU(2)_L x U(1)_Y singlet and
compute the electroweak precision parameter Delta r, along with the
corresponding theoretical prediction for the W-boson mass. When confronted with
the experimental W-boson mass measurement, our predictions impose limits on the
singlet model parameter space. We identify regions where these correspond to
the most stringent experimental constraints that are currently available.
