Interesting facts can be found in the following source:
„White emigrant in Bulgaria. Memories“ under redaction of V.V. Chumachenko, P.V. Chumachenko, S.A. Rozhkova,
published by the russian academic union in Bulgaria.
The PDF-file has nearly 800 pages covering the russian emigration to Bulgaria including some biographical data. In my case I had a look on a part about the russian schools in Sofia / Bulgaria. In the caption to the pictures documentation the schools you will find the name „Goyer = Hoyer“ quite often..
The first surname (Oleg) was a direct hit, the second one (Juri) needed a little more mental work…Juri ? Maybe there has been another child not known to me ?
On second thoughts I remembered that Juri is the russian form of the german name of „Georg“ and now we have the two brothers gogether.
Caption: Teacher and pupils of the residential school for boys, School year 1926/1927
10 – Гойер Юрий = Hoyer Juri
25 – Гойер Олег = Hoyer Oleg
Caption:New orchestra of the secondary school. School year 1931/1932
6 – Гойер Юрий = Hoyer Juri
Caption: 3rd and 4th-classes. School year 1927/1928
11 – Гойер Юрий = Hoyer Juri
Caption: Visit of Mr. Pavel Krylov (24.05.1934), Graduating day ?
10 – Гойер Юрий = Hoyer Juri
Conclusion: The time in Bulgaria seems to be well documented, but there is the third brother missing (Nicolai, born 1906). I can only suggest that we was too old at that time for the same education as his younger brothers…