  A union ultrafilter is an ultrafilter over the finite subsets of $\omega$
that has a base of sets of the form $\mathrm{FU}(X)$, where $X$ is an infinite
pairwise disjoint family and $\mathrm{FU}(X)=\{\bigcup
F\big|F\in[X]^{<\omega}\setminus\{\varnothing\}\}$. The existence of these
ultrafilters is not provable from the $\mathsf{ZFC}$ axioms, but is known to
follow from the assumption that $\mathrm{cov}(\mathcal{M})=\mathfrak c$. In
this article we obtain various models of $\mathsf{ZFC}$ that satisfy the
existence of union ultrafilters while at the same time
$\mathrm{cov}(\mathcal{M})<\mathfrak c$.
