  We consider the problem of reproducing the correlations obtained by arbitrary
local projective measurements on the two-qubit Werner state $\rho = v |\psi_- >
<\psi_- | + (1- v ) \frac{1}{4}$ via a local hidden variable (LHV) model, where
$|\psi_- >$ denotes the singlet state. We show analytically that these
correlations are local for $ v = 999\times689\times{10^{-6}}$ $\cos^4(\pi/50)
\simeq 0.6829$. In turn, as this problem is closely related to a purely
mathematical one formulated by Grothendieck, our result implies a new bound on
the Grothendieck constant $K_G(3) \leq 1/v \simeq 1.4644$. We also present a
LHV model for reproducing the statistics of arbitrary POVMs on the Werner state
for $v \simeq 0.4553$. The techniques we develop can be adapted to construct
LHV models for other entangled states, as well as bounding other Grothendieck
constants.
