Higgs boson form factor effects in $tbar t$ production by $W^-W^+$ and $ZZ$ fusion

We study the fusion processes $W^-W^+to tbar t$ and $ZZto tbar t$ observable at a future $e^-e^+$ collider and we discuss their sensitivity to an $Htt$ form factor which may be due to compositeness, in particular when the $H$ and the top quark have common constituents. We make an amplitude analysis and illustrate which helicity amplitudes and cross sections for specific final $tbar t$ polarizations are especially sensitive to this form factor.