# Misplaced accent in ec-lmr10

In plain TeX, \ddot{a} places the double-dot accent correctly above the letter a. However, if I (attempt to) switch from the default (Computer Modern) fonts to ec-lmr10 (I believe that this is Latin Modern?), the double dot accent ends up in the middle of a.

Running pdftex on the code below gives me two correct "double-dotted a", two correct "vectored a", then two wrong "double-dotted a" and two correct "vectored a", so the problem depends on the accent.

$\ddot{a}\mathaccent"707F{a}$
$\vec{a} \mathaccent"017E{a}$
\font\tenrm = ec-lmr10 at 10pt\relax
\textfont0 = \tenrm
$\ddot{a}\mathaccent"707F{a}$
$\vec{a} \mathaccent"017E{a}$
\bye


Two questions: What is causing the diffference? (Some fontdimen parameter, perhaps?) And what am I doing wrong, how could I fix that?

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The font ec-lmr10 is T1-encoded, not OT1. So at position "7F it hasn't the dieresis.
With \mathaccent"7004 you get the right accent.