  We study the competition between local (bridging) and global condensation of
fluid in a chemically heterogeneous capillary slit made from two parallel
adjacent walls each patterned with a single stripe. Using a mesoscopic modified
Kelvin equation, which determines the shape of the menisci pinned at the stripe
edges in the bridge phase, we determine the conditions under which the local
bridging transition precedes capillary condensation as the pressure (or
chemical potential) is increased. Provided the contact angle of the stripe is
less than that of the outer wall we show that triple points, where evaporated,
locally condensed and globally condensed states all coexist are possible
depending on the value of the aspect ratio $a=L/H$ where $H$ is the stripe
width and $L$ the wall separation. In particular, for a capillary made from
completely dry walls patterned with completely wet stripes the condition for
the triple point occurs when the aspect ratio takes its maximum possible value
$8/\pi$. These predictions are tested using a fully microscopic classical
Density Functional Theory and shown to be remarkably accurate even for
molecularly narrow slits. The qualitative differences with local and global
condensation in heterogeneous cylindrical pores are also highlighted.
