 The study is designed to provide data on the efficacy, safety and cognitive properties of Lu AE58054 as augmentation therapy to risperidone in patients with schizophrenia. Efficacy will be assessed in patients who are in a stable phase of their illness, but with a predefined minimum and maximum level of symptoms that will allow them to be included in the study. Patients will be randomly assigned to receive either the investigational medicinal product IMP or placebo as add-on therapy to their existing risperidone treatment. 