Two events $A$ and $B$ are mutually exclusive, and
$$\mathrm { P } ( A ) = \mathrm { P } ( B ) , \quad \mathrm { P } ( A ) \mathrm { P } ( B ) = \frac { 1 } { 9 }$$
What is the value of $\mathrm { P } ( A \cup B )$? [3 points]\\
(1) $\frac { 1 } { 6 }$\\
(2) $\frac { 1 } { 3 }$\\
(3) $\frac { 1 } { 2 }$\\
(4) $\frac { 2 } { 3 }$\\
(5) $\frac { 5 } { 6 }$