Title: On weak Fraisse limits,
Abstract: Using the natural action of $S_\infty$ we show that a countable hereditary
class $\cC$ of finitely generated structures has the joint embedding property
(JEP) and the weak amalgamation property (WAP) if and only if there is a
structure $M$ whose isomorphism type is comeager in the space of all countable,
infinitely generated structures with age in $\cC$. In this case, $M$ is the
weak Fraïssé limit of $\cC$.
This applies in particular to countable structures with generic automorphisms
and recovers a result by Kechris and Rosendal [\textit{Proc.\ Lond.\ Math.\
Soc.,\ 2007}].