1. Introduction  
2. Observations  
2.1. Spectroscopy  
2.2. Photometry  
3. Analysis  
3.1. Radial velocities and the orbital solution  
3.2. Atmospheric parameters  
3.3. Light curve solution  
4. Discussion  
4.1. Activity  
4.2. Kinematics  
4.3. Comparison with theoretical models  
4.4.  $0.4-0.9\,M_{\odot}$ stars in DEBs  
5. Summary