The Mercator telescope is a 1.2m semi-automatic telescope placed at the Roque de los Muchachos observatory at the Canary Islands. The permanent availability allows us to occupy a specific niche in observational astrophysics: the possibility to make long-term time-series. To exploit this fully we developed a modern instrument programme. I will start this contribution with a review of our world-class instruments HERMES (a high-resolution optical spectrograph with a high throughput) as well as MAIA (3-arm camera with large fame-transfer CCDs). By continuous innovation, we maintain our instrument suite and telescope as state of-the art facilities. In the main part of the contribution I will illustrate our scientific research based on Mercator data by deploying several themes focussed on our research on stellar structure and evolution of single and binary stars.