Balkan mountain range
Varna
Its the Danube river and Danube plain separates Romania and Bulgaria. There are no mountaneous border between the mentioned two countries. And Carpathian mountain range of Romnia or Balkan mountain range (most northern mountains of Bulgaria: none of them serves as border.
The Balkan Mountains run through the center of Bulgaria. They are the longest mountain range in Bulgaria and divide the country into northern and southern regions.
The Balkan Mountain range (in Bulgaria Balkan Mountain is often called the 'Stara Planina' Mountains)
The Stara planina mountain (Стара планина)
Balkan mountain range, The Daniube and a capital Sofia!
Stara Planina
The Balkan mountain range runs from the border with Serbia 560km eastwards to the Black Sea
A common mountain range covering Bulgaria and Serbia is called the Balkan Mountains (in Bulgarian also called 'Stara Planina')
The Summit of the Rila Mountain range Musala is at height-9596 feet
Balkan Mountain ranges (Stara Planina), biggest range covering Bulgaria and Serbia, Rhodopes range (Rodopi), Rila, Pirin. and may be Vitosha (some includes it Balkan range!)