  We study the process of single-top production in association with a charged
Higgs boson at the LHC and discuss how top-quark polarization and an azimuthal
asymmetry $A_\varphi$ of the charged lepton from top decay can be used in some
extensions of the Standard Model to determine or constrain the charged Higgs
boson mass. We also discuss some scenarios where these variables can be used to
distinguish between different models.
