[XeTeX] How to specify a strict leading space with XeTeX/fontspec and Memoir class?

[stefano franchi]

I am just switching from Latex to XeLatex, and I cannot quite understand how
to force it to use a specific interline spacing (i.e. "leading") in certain
occasions.
More precisely:
I am using the memoir class, with a rather heavy customization.
At publisher demand,  I need to have all chapter titles, chapter numbers,
etc, in Times (or equivalent) at exactly 15pt  with  18pt leading
The fontsize command did the trick in memoir using Latex:

\renewcommand*{\chaptitlefont}{\fontfamily{ptm}\fontsize{15}{18}
\selectfont}


But I do not understand how to replicate the same behavior in XeLatex.

I can create a 15 pt Times-like  font command with

 \font\VIBSChapterTitleFont="TeX Gyre Termes:letterspace=3" at 15pt


and then set the chapter title font command to it:

\renewcommand*{\chaptitlefont}{\VIBSChapterTitleFont}


But how can I control the leading space? I tried using the leading package,
but it produced  very strange results (it changed the font size).


Sorry if I sound confused. I really am.


All help appreciated.


Cheers,

Stefano
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