We describe the positive cone and the pseudo-effective cone of a
non-K"ahlerian surface. We use these results for two types of applications:
- Describe the set σ(X) of possible total Ricci scalars associated
with Gauduchon metrics of fixed volume 1 on a fixed non-K"ahhlerian surface,
and decide whether the assignment X↦σ(X) is a deformation
invariant.
- Study the stability of the canonical extension
0→KX→A→OX→0 of a class VII surface X
with positive b2. This extension plays an important role in our strategy to
prove the GSS conjecture using gauge theoretical methods.