Update on $B o D^ast ell u$ form factor at zero-recoil using the Oktay-Kronfeld action.

We present an update on the calculation of $bar{B}to D^ast ell bar{nu}$ semileptonic form factor at zero recoil using the Oktay-Kronfeld bottom and charm quarks on $N_f=2+1+1$ flavor HISQ ensembles generated by the MILC collaboration. Preliminary results are given for two ensembles with $aapprox 0.12$ and $0.09$ fm and $M_piapprox 310$ MeV. Calculations have been done with a number of valence quark masses, and the dependence of the form factor on them is investigated on the $aapprox 0.12$ fm ensemble. The excited state is controlled by using multistate fits to the three-point correlators measured at 4--6 source-sink separations.