1. Introduction  
1.1. The YMC Westerlund 1  
2. Data acquisition and reduction  
2.1. Spectroscopy  
2.2. Photometry  
3. Observational properties and cluster membership  
3.1. Spectral classification  
3.2. Spectral variability  
3.3. Cluster membership  
4. Quantitative modeling  
4.1. Methodology  
4.2. Wd1-27 results  
4.3. Wd1-30a results  
5. Discussion  
5.1. Stellar evolution in Wd1  
5.2. Massive blue stragglers in other stellar aggregates?  
6. Conclusions