We compare Stein fillings and Milnor fibers for rational surface singularities with
reduced fundamental cycle. Deformation theory for this class of singularities was studied
by de Jong and van Straten (1998); they associated a germ of a singular plane curve
to each singularity and described Milnor fibers via deformations of this singular curve.
We consider links of surface singularities, equipped with their canonical contact
structures, and develop a symplectic analog of de Jong and van Straten’s
construction. Using planar open books and Lefschetz fibrations, we describe all Stein
fillings of the links via certain arrangements of symplectic disks, related by a
homotopy to the plane curve germ of the singularity.
As a consequence, we show that many rational singularities in this class
admit Stein fillings that are not strongly diffeomorphic to any Milnor fibers.
This contrasts with previously known cases, such as simple and quotient
surface singularities, where Milnor fibers are known to give rise to all Stein
fillings. On the other hand, we show that if for a singularity with reduced
fundamental cycle, the self-intersection of each exceptional curve is at most
in the
minimal resolution, then the link has a unique Stein filling (given by a Milnor
fiber).