markdown's bold and italic stop working after including babel package

Question

I'm having problems with bold and italic options in my pdf file that I get from my Rmarkdown file. I write in Russian so I unclude babel package and after that ** and __ stop working.

Here's my Rmarkdown file. The result is in plain font, the word is not bold.

---
output: pdf_document
header-includes:
- \usepackage[russian]{babel}
---
**Theorem**.

Here is result of convertion of this Rmd-file to tex.

While this version keeps the word bold:

---
output: pdf_document
---
**Theorem**

I also tried to change latex engine from pdflatex to xelatex but for xelatex I also should specify some special font that supports cyrillic. If I don't specify mainfont there's no cyrillic letters in the result. And I would really like to stay with the default font.

This works but I don't like the font:

---
output:
  pdf_document:
    latex_engine: xelatex
mainfont: Arial
---

**Теорема**.

This doesn't work:

---
output:
  pdf_document:
    latex_engine: xelatex
---

**Теорема**.

Answer 1

Recommendation from @Ulrike Fischer worked: I found default.tex used by Rstudio as a template when converting from Rmd to pdf (~/R/x86_64-pc-linux-gnu-library/3.2/rmarkdown/rmd/latex/default.tex) and removed \usepackage{lmodern} from there. After that this piece of code compiled correctly:

---
output: pdf_document
header-includes:
- \usepackage[russian]{babel}
---
**Theorem**.