The equivalent capacitance between A and B is

For the arrangement of capacitors as shown in the circuit, the effective capacitance between the points $A$ and $B$ is (capacitance of each capacitor is $4 \mu \mathrm{~F}$ )

Two capacitors of $3 \mu \mathrm{~F}$ and $6 \mu \mathrm{~F}$ are connected in series and a potential difference of 900 V is applied across the combination. They are then disconnected and reconnected in parallel. The potential difference across the combination is