Surfaces on the Severi line

Let S be a minimal complex surface of general type and of maximal Albanese dimension; by the Severi inequality one has KS2 ≥ 4χ(OS). We prove that the equality KS2 = 4χ(OS) holds if and only if q(S) := h1(OS) = 2 and the canonical model of S is a double cover of the Albanese surface branched on an ample divisor with at most negligible singularities.