What do the following word have in common?

1. HER

2. LOG

3. VET

4. SUM

 

HINT:

PROM and COMB could be added to the list.

 

Answer:

 

Each word becomes a new one with the addition of O at the end:

 

1. HERO

2. LOGO

3. VETO

4. SUMO 

(PROMO and COMBO)