On surfaces with pg = q=1 and non-ruled bicanonical involution

This paper classifies surfaces S of general type with p(g)=q=1 having an involution i such that S/i has non-negative Kodaira dimension and that the bicanonical map of S factors through the double cover induced by i. It is shown that S/i is regular and either: a) the Albanese fibration of S is of genus 2 or b) S has no genus 2 fibration and S/i is birational to a K3 surface. For case a) a list of possibilities and examples are given. An example for case b) with K-2=6 is also constructed.