Running the following MWE
\documentclass{article}
\begin{document}
Test $\mathrm{VARfi}$
\end{document}
using XeLaTeX or LuaLaTeX, I observed that Test was rendered in OpenType Latin Modern Roman while VAR was rendered in TFM-based Computer Modern Roman. Is it possible to change \mathrm to use OpenType Latin Modern Roman?
VandAand thefiligature will both disappear, and therefore the result is not the full OpenType Latin Modern Roman. – Ruixi Zhang Oct 22 '19 at 04:00fontspecpackage, one also needs to execute\setmainfont{Latin Modern Roman}to get your desired outcome, viz., kerning between V and A and thefiligature. – Mico Oct 22 '19 at 04:45:(. – Ruixi Zhang Oct 22 '19 at 15:32$\mathrm{VARfi}$need more works and XeTeX refuses to kern or to join letters into ligatures as far as I know. – Ruixi Zhang Oct 22 '19 at 15:34