Common Zebra Blue - Leptotes pirithous
Lang's Short-tailed Blue - most probably this species is migrant and doesn't have permanent populations in Bulgaria.
I chased a couple of them on the Danube river bank. They always used to land on the ground only for a couple of seconds and I had to hover in the mud in order to get a better point of view. Quite frustrating....
''Leptotes pirithous'', the Lang's short-tailed blue or common zebra blue, is a butterfly of the family Lycaenidae.