  We investigate the scaling behaviour of quenched Wilson twisted mass fermions
at maximal twist applying two definitions of the critical mass. The first
definition uses the vanishing of the pseudoscalar meson mass m_PS while the
second employs the vanishing of the PCAC quark mass m_PCAC. We confirm in both
cases the expected O(a) improvement. In addition, we show that the PCAC quark
mass definition leads to substantially reduced O(a^2) cut-off effects even when
the pseudoscalar meson mass m_PS is as small as 270 MeV. At a fixed value of
m_PS we perform continuum limits for the vector meson mass m_V and for the
pseudoscalar decay constant f_PS and discuss the renormalisation constant Z_V
of the vector current.
