Make the filling by first beating the eggs in a bowl, then add the yoghurt and oil, stir, crumble in the feta cheese and add the carbonated water.
Lay out the phyllo pastry sheets on the table lengthwise. Grab 2 sheets at a time, sprinkle filling on and distribute it all over the entire surface, then fold in 2, smear with filling again and fold one final time into a triangle.
Arrange all of the pastries in an oven dish laid out with baking paper. Put a small piece of butter on each one. Bake them in a preheated 356°F (180 °C) oven for about 30 min. until golden.