  The universality of Martinez's conjecture, which states that the quasilocal
energy of a black hole at the outer horizon reduces to twice its irreducible
mass, or equivalently, to $\sqrt{A/4\pi}$ ($A$ is the area of the black hole),
is investigated by calculating Brown-York quasilocal energies for stationary
black holes in heterotic string theory, e. g., for the stationary Kaluza-Klein
black hole, the rotating Cveti$\check{c}$-Youm black hole, the stationary
axisymmetric Einstein-Maxwell-dilaton-axion black hole, and the Kerr-Sen black
hole. It is shown that Martinez's conjecture can be extended from general
relativity to heterotic string theory since the quasilocal energies of these
stationary black holes tend to their Arnowitt-Dener-Misner masses at spatial
infinity, and reduce to $\sqrt{A/4\pi}$ at the event horizons.
