Yes it helps, thanks; it explains why there was so little response to this. ;)
After some search I found regular style pictures of it. I was sure, that it was italic as standard, because that's what "modern" suggests: compability for current regulations (i.e. regular constants, italic variables) as well as an attractive style overall. But since Neo Euler seems to have no italic style, it is of no use for academic purposes, like in my case.

The story, to add to the subject of the mail, in case somebody searches the archive for this, is that I wasn't able to find out is how to change the math font only, as i like the standard font with \setupbodyfont[sans] as global setup but not the style of (non-modern) Euler.
I tried

\definefontfamily [mainface] [mm] [Xits Math]
but this changes the whole typeface; not what I want.
So the trick is to define the standard font for the rest of the font variations. The standard font can be found out with \showbodyfont. At least that's what it suggests. But it gives you "modern-design" as font. When you use this within \definefontfamily then it gives you nothing, because the real id of the standard font is "modern". So to change the math font only and keep the main font standard, you have to use this command group (just in case somebody looks for it):
\definefontfamily [mainface] [rm] [Modern] % = serif
\definefontfamily [mainface] [ss] [Modern] % = sans
\definefontfamily [mainface] [tt] [Modern] % = mono (teletype)
\definefontfamily [mainface] [mm] [Xits Math] [rscale=1.03] % = math

But as this seems to be so trivial, that few people care, I really have to wonder why people who test CTX criticise the lack of documentation. We (especially who haven't been through LaTeX or even TeX) seem to be a minority.

Cheers and thanks, S.
On 23/07/2017 21:22, Pablo Rodriguez wrote:
On 07/22/2017 03:16 AM, Sebastian L. wrote:

Hi, I have trouble with Neo Euler font after playing around with those
options.
[...]
I am pretty sure that before I started mixing the fonts in one single
document, Neo Euler was italic.

Does anybody have a clue what might have went wrong?

Hi Sebastian,

as far as I remember, Neo Euler is a regular font only
(https://github.com/khaledhosny/euler-otf).

Euler is an italic font and it might have the other regular, bold, bold
italic typefaces (I don’t know).

I hope it helps,

Pablo