A $375$W, $230$V, $50$Hz, capacitor start single-phase induction motor has the following constants for the main and auxiliary windings (at starting): $Z_{m}=(12.50+j15.75) \Omega$ (main winding), $Z_{a}=(24.50+j12.75) \Omega$ (auxiliary winding). Neglecting the magnetising branch, the value of the capacitance (in $\mu F$) to be added in series with the auxiliary winding to obtain maximum torque at starting is ________.