  We investigate Wilson twisted mass fermions in the quenched approximation
using different definitions of the critical bare quark mass m_c to realize
maximal twist and, correspondingly, automatic O(a) improvement for physical
observables. A particular definition of m_c is given by extrapolating the value
of m_c obtained from the PCAC relation at non-vanishing bare twisted quark mass
mu to mu=0. Employing this improved definition of the critical mass the Wilson
twisted mass formulation provides the possibility to perform reliable
simulations down to very small quark masses with correspondingly small pion
masses of m_pi \simeq 250 MeV, while keeping the cutoff effects of O(a^2) under
control.
