My fathers ancestors from Bulgaria

Hello from Austria,
my Grandmother Sorka Soritza Maria Petrowa - Katerowa FRANTZ
was Born 22. September 1900 Chirpan, Bulgaria
died 13. November 1979 Linz, Austria
Parents:
Peter Nejkov Katerov
Born 1870 unkn.
died Sept. 1951 Sofia, Bulgaria
and
Maria Marijka Petrowa Katerowa
Born unkn. Chirpan, Bulgaria
died Feb. 1952 Sofia, Bulgaria
Would be great to find out more about my Bulgarian ancestors.
Best regards !
Charlotte