We present a preliminary analysis of time-resolved X-shooter spectroscopy of HW Vir, a post-common envelope binary system, comprising of hot subdwarf B primary and a cool M dwarf secondary. Aiming at detecting and analysing the irradiated light of the superheated hemisphere of the M dwarf, we demonstrate that HW Vir is a bona fide double lined system allowing us to directly derive the masses of both stars and consequently backtrack the controversial evolution of subdwarf B stars and test the mass-radius relation of low mass stars.