A common mountain range covering Bulgaria and Serbia is called the Balkan Mountains (in Bulgarian also called 'Stara Planina')
The Balkan mountain range runs from the border with Serbia 560km eastwards to the Black Sea
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!)
Balkan mountain range
There are two mountain ranges extended beyond the frontier of Bulgaria.a. The Balkan Mountain Range (local name- Stara Planina), extended deep in Serbia. Its in the northern half of Bulgaria. Direction: North West proceedingb. The Rhodopes Mountain Range (local name- Rodopi), passed inside Greece. Its in the Southern half of Bulgaria. Direction: South East proceeding
No mountain in the Daniube Plain. A big range of mountains are available if one goes around 100 km southwards. The mountain is Known as Balkan mountain. The local people calles it Stara Planina. Its the biggest mount. range by lenghth in Bulgaia..and extended to Serbia
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.
Bulgaria.
Serbia
The Stara planina mountain (Стара планина)