Don't know if this could help.....but when I visited Yugoslavia in 1977, my relatives called me Radojka or Rada...but when they were in a "fun"-happy mood, they'd call me Rajko.
My mom still calls me Rajko an a repeated basis, especially when I call her on the phone, "oh, toh moja Rajko meh zoveh" when she finds out it's me calling ((sorry the spelling is not correct, but phonetically it sounds correct...I was born in the USA, and was never formally taught to read/write in our language)) My apologies.
Something else to add... While growing up, I also knew of a few men having the first name as Rade and they were also known as Robert in English/American.And since so many family members called me Rada and these family members said that Rade is the masculine form of Rada.....then I just always thought that maybe Radojka would translate into English as Roberta...(glad I don't go by that for real)
oh well.Don't know if that helped, but hey, I tried.