The Wood White (Leptidea sinapis) looks very similar to the Cryptic Wood White (Leptidea juvernica) and the Réal's Wood White (Leptidea reali).
All three species always rests with their wings against each other, which makes their upper surface difficult to study. Their long, round-tipped wings and slow, fluttering flying style are characteristic properties of these three wood white species, and it is very difficult to tell them apart with absolute certainty. The only conclusive way to separate the three species is by close examination of the genitalia and DNA analysis.
However, Réal's Wood White is found in Italy, Spain and France so the species on the photo is either a Wood White or a Cryptic Wood White. Both species are found in the Balkan countries, including Bulgaria.