  PSR J1734-3333 is a radio pulsar rotating with a period P=1.17 s and slowing
down with a period derivative Pdot=2.28 x 10^{-12}, the third largest among
rotation-powered pulsars. These properties are midway between those of normal
rotation-powered pulsars and magnetars, two populations of neutron stars that
are notably different in their emission properties. Here we report on the
measurement of the second period derivative of the rotation of PSR J1734-3333
and calculate a braking index n=0.9 +- 0.2. This value is well below 3, the
value expected for an electromagnetic braking due to a constant magnetic
dipole, and indicates that this pulsar may soon have the rotational properties
of a magnetar. While there are several mechanisms which could lead to such a
low braking index, we discuss this observation, together with the properties
exhibited by some other high-Pdot rotation-powered pulsars, and interpret it as
evidence of a possible evolutionary route for magnetars through a radio-pulsar
phase, supporting a unified description of the two classes of object.
