5. Given the circle $\odot O$ with radius 1, the angle between radii $O A$ and $O B$ is $\varphi(0<\varphi<\pi, \varphi$ is a constant $), C$ is a moving point on the circle. If $\overrightarrow{O C}=x \overrightarrow{O A}+y \overrightarrow{O B}(x, y \in \mathbf{R})$, then the maximum value of $x+y$ is $\qquad$.
$\frac{1}{\cos \frac{\varphi}{2}}$