Moving underset text closer to main text

Question

I have an arrow with some text directly underneath, using the command

\newcommand*{\tautrans}%
  {\underset{\scriptstyle \tau^*}{\longrightarrow}}%

Since \tau^* is not close enough to \longrightarrow, I changed to

\newcommand*{\tautrans}%
  {\underset{\scriptstyle \raisebox{.2em}{\ensuremath{\tau^*}}}{\longrightarrow}}%

Unfortunately, this changes the distance between lines, but not between \tau^* and \longrightarrow.

So how can I move \tau^* a little bit up without moving anything else, so that \tau^* comes closer to \longrightarrow?

Answer 1

Here's a possible solution. I defined \tautransi using \xrightarrow; the text under the arrow can be raised using a \raisebox (change the length according to your needs). The code shows a comparison between \tautrans and \tautransi

\documentclass{article}
\usepackage{amsmath}

\newcommand*{\tautrans}{%
  \underset{\tau^*}{\longrightarrow}}
\newcommand*{\tautransi}{%
  \xrightarrow[{\raisebox{.4ex}[0pt][0pt]{$\scriptstyle\tau^*$}}]{\quad}}

\begin{document}

\[
\tautrans\quad\tautransi
\]

\end{document}