1. Introduction  
2. Sample Selection and He II Emitter Identification  
2.1. Line Identification  
2.2. Line Measurements of Bright Sources  
2.3. Possible AGNs in the Sample  
2.4. Stacking  
3. Physical Properties of He II Emitters  
3.1. Stellar Masses  
3.2. Star Formation Rates  
3.3. Specific SFR  
3.4. Rest-frame UV Magnitudes  
4. Comparison with Photo-ionisation Models  
4.1. Comparison with Gutkin et al. (2016)  
4.2. Comparison with Xiao et al. (2018) – Binary Stellar Population  
4.3. Revisiting the He II Emitters with Strong C IV  
4.4. Effects of Different Stellar Population Parameters  
5. Discussion  
5.1. Do He II-emitting Galaxies Have Systematically Lower Metallicities?  
5.2. Alternative Sources of He II Ionising Photons  
6. Conclusions