Toss the potatoes, zucchini, and salt in a medium bowl; let stand 10 minutes.

Squeeze out the liquid and discsard.

Stir the egg, scallions, flour, tarragon, and pepper into the potato mixture.

Heat 1/4 teaspoon of the oil in a large nonstick skillet over medium heat.

Drop the potato mixture, 2 tablespoons at a time, into mounds and flatten with a spatula.

Cook until the bottoms are lightl browned, about 6 minutes.

Turn the pancakes over, add 1/4 teaspoon of the oil, and cook about 6 minutes.

Repeat with the remaining mixture to make 8 pancakes.

Serve with the sour cream.