1. Introduction
2. Dataset
2.1. MUSE HUDF Survey and sample selection
2.2. Properties of LAEs
2.3. Spectral extractions
3. Spectral stacking
3.1. Stacking methodology
3.2. Stacked spectra of LAEs
3.3. Velocity offset correction
4. Results
4.1. O iii ], C iii ], Si iii ], and He ii lines
4.2. CIV doublet and absorption features
5. Discussion
5.1. Dependence of UV line features on galaxy properties
5.2. Comparison with the literature
5.3. AGN and other ionizing sources
6. Summary and Conclusions