  In this note we obtain a version of the well-known Riesz's theorem on
conjugate harmonic functions for Lumer's Hardy spaces $(Lh)^2(\Omega)$ on
arbitrary domains $\Omega$: If a real-valued harmonic function $U\in
(Lh)^2(\Omega)$ has a harmonic conjugate $V$ on $\Omega$ (i.e., a real-valued
harmonic function such that $U+ iV$ is analytic on $\Omega$), then $U+iV$ also
belongs to $(Lh)^2(\Omega)$, and for the normalized conjugate we have the norm
estimate $\|U+iV\|_{(Lh)^2(\Omega)}\le\sqrt{2} \|U\|_{(Lh)^2(\Omega)}$, with
the best possible constant.
