  We study the effects of an integrable impurity in a periodic t-J chain. The
impurity couples to both spin and charge degrees of freedom and has the
interesting feature that the interaction with the bulk can be varied
continuously without losing integrability. We first consider ground state
properties close to half-filling in the presence of a small bulk magnetic
field. We calculate the impurity contributions to the (zero temperature)
susceptibilities and the low temperature specific heat and determine the
high-temperature characteristics of the impurity. We then investigate transport
properties by computing the spin and charge stiffnesses at zero temperature.
Finally the impurity phase--shifts are calculated and the existence of an
impurity bound state in the holon sector is established.
