I picked up a nifty receptacle/GFCI tester, and was surprised that a
number of outlets in the older part of our house (most of the house is
a later addition) are not grounded.

Simple enough. I shut off the breaker, pulled the faceplate and the
outlet out of the box, and intended to connect the line grounding wire
to the screw on the outlet.

After stripping back the sheathing, I realized that the feed cable is
not equipped with a green or bare copper grounding wire. There's just
a black and white.

My question is, short of rewiring the circuit, how do I ground these
outlets?