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$\gamma$ Cas stars are a $\sim$1% minority among classical Be stars with hardbut only moderately strong continuous thermal X-ray flux and mostly veryearly-B spectral type. The X-ray flux has been suggested to originate frommatter accelerated via magnetic disk-star interaction, by a rapidly rotatingneutron star (NS) companion via the propeller effect, or by accretion onto awhite dwarf (WD) companion. In view of the growing number of identified$\gamma$ Cas stars and the only imperfect matches between these suggestions andthe observations, alternative models should be pursued. Two of the threebest-observed $\gamma$ Cas stars, $\gamma$ Cas itself and $\pi$ Aqr, have alow-mass companion with low optical flux; interferometry of BZ Cru isinconclusive. Binary-evolution models are examined for their ability to producesuch systems. The OB+He-star stage of post-mass transfer binaries, which isotherwise observationally unaccounted, can potentially reproduce many observedproperties of $\gamma$ Cas stars. The interaction of the fast wind of heliumstars with the disk and/or with the wind of Be stars may give rise to theproduction of hard X-rays. While not modelling this process, it is shown thatthe energy budget is favourable, and that the wind velocities may lead to hardX-rays as observed in $\gamma$ Cas stars. Furthermore, their observed numberappears to be consistent with the evolutionary models. Within the Be+He-starbinary model, the Be stars in $\gamma$ Cas stars are conventional classical Bestars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass,where no stable Be decretion disks exist, and by Be+sdO systems at lower masswhere the sdO winds may be too weak to cause the $\gamma$ Cas phenomenon. Indecreasing order of the helium-star mass, the descendants could beBe+black-hole, Be+NS or Be+WD binaries.

gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?