Let F be a polynomial in two variables with integer coefficients, let D be its degree and let M >= 3 be an upper bound for the absolute value of its coefficients. Then the number of rational zeroes of F is either infinite or less than exp(5(D4) (log M)(log log M)). We prove this as a special case of a result for number fields. The main new ingredient is an estimate for the theta height of a Jacobian.