Minimal generator for a set of sets Is this a known problem?

Given a set of sets $S$ find a set of sets $B$ s.t. each set in $S$ can be obtained through unions of some sets in $B$. The set $S$ is already a solution but the objective is to find the smallest $B$. 

For instance $\{\{a,b\}, \{c\}, \{a,b,c\}\}$ can be obtained from $\{\{a,b\}, \{c\}\}$.

Since it's somewhat similar to set-cover, I suspect it's NP-hard.

Additional remark: $B$ does not have to be a subset of $S$.
A decision variant, without the minimality condition, asking whether there is a set $B$ of size $n$ is called the set basis problem [SP7] in Computers and Intractability: A Guide to the Theory of NP-Completeness by Garey and Johnson. The NP-completeness of it was proved by Stockmeyer.