Two pipes $P_1$ and $P_2$ fill a tank. The set of times $T_1$ for $P_1$ to fill is $\{2, 4, 6\}$ hours and $T_2$ for $P_2$ is $\{3, 6, 9\}$ hours. If $t_1 \in T_1$ and $t_2 \in T_2$ are chosen such that they work together, what is the set of possible times $t = \frac{t_1 t_2}{t_1 + t_2}$ if both are chosen from the intersection $T_1 \cap T_2$?