We prove the existence of nontrivial rational Heegner points on the elliptic curve $y^2 = x^3 + p$ when $p \equiv 5 \pmod{36}$ and $h_K$ is odd, and on the elliptic curve $y^2 = x^3 - p$ when $p \equiv 23 \pmod{36}$ and $h_K$ is odd. This follows from our expression for the fundamental unit of the non-Galois cubic extension $K=\mathbb{Q}(\sqrt[3]{p})$ in terms of the class number $h_K$ and the norm of a special value of a modular function of level $6$ for any odd prime $p \equiv 2$ or $5 \pmod{9}$, which is an analogue of a theorem of Dirichlet in 1840 expressing the fundamental unit of a real quadratic field in terms of the class number and a product of cyclotomic units.