how to merge 2 letters (in math mode)

Question

I have a special operator which is now denoted using 2-letter name MR. I would like to merge them into special symbol where these two letters merged, sharing vertical bar. Similar to Æ.

Answer 1

Something like this? The negative kerning values are a guess, however. (Improvement suggestions by Werner and egreg, many thanks to both of them!)

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\mr}{M\MRkern R}
\newcommand{\MRkern}{%
  \mkern-6.5mu
  \mathchoice{}{}{\mkern0.2mu}{\mkern0.5mu}%
}

\begin{document}

$\mr$ $\scriptstyle\mr$ $\scriptscriptstyle\mr$

\end{document}