I'm probably using the \starttable incorrectly:

with this code I only get only *one* horizontal line in the header

\starttable[|r|r|r|r|r|r|r|l|r|r|r|r|][bodyfont=9pt]
\NC step\NC frac\NC ival\NC  \NC\Use{4}[cI]{freq 
(Hz)}\NC~\NC\Use{4}[cI]{wavelength (m)}\AR
\NC\DC\DC\DC\DL[4]\DL[4]\AR
\NC 0 \NC 0.00 \NC 0.000 \NC 12.50\NC 13.75\NC 15.00\NC 12.25\NC A\ \NC 
27.20\NC 24.73\NC 22.67\NC 27.76\AR
\NC 1 \NC 0.08 \NC 0.115 \NC 13.94\NC 15.34\NC 16.73\NC 13.66\NC A\#\NC 
24.38\NC 22.17\NC 20.32\NC 24.88\AR
\NC 2 \NC 0.17 \NC 0.222 \NC 15.28\NC 16.81\NC    18.34\NC    14.97\NC 
B\ \NC 22.25\NC 20.23\NC 18.54\NC 22.71\AR
\stoptable




with this code I get two horizontal lines, but the second is too short.

\starttable[|r|r|r|r|r|r|r|l|r|r|r|r|][bodyfont=9pt]
\NC step\NC frac\NC ival\NC  \NC\Use{4}[cI]{freq 
(Hz)}\NC~\NC\Use{4}[cI]{wavelength (m)}\AR
\NC\DC\DC\DC\DL[4]\DL[3]\AR
\NC 0 \NC 0.00 \NC 0.000 \NC 12.50\NC 13.75\NC 15.00\NC 12.25\NC A\ \NC 
27.20\NC 24.73\NC 22.67\NC 27.76\AR
\NC 1 \NC 0.08 \NC 0.115 \NC 13.94\NC 15.34\NC 16.73\NC 13.66\NC A\#\NC 
24.38\NC 22.17\NC 20.32\NC 24.88\AR
\NC 2 \NC 0.17 \NC 0.222 \NC 15.28\NC 16.81\NC    18.34\NC    14.97\NC 
B\ \NC 22.25\NC 20.23\NC 18.54\NC 22.71\AR
\stoptable


with this code I get two horizontal lines, with correct length, but 
above the wrong columns

\starttable[|r|r|r|r|r|r|r|l|r|r|r|r|][bodyfont=9pt]
\NC step\NC frac\NC ival\NC  \NC\Use{4}[cI]{freq 
(Hz)}\NC~\NC\Use{4}[cI]{wavelength (m)}\AR
\NC\DC\DC\DL[4]\DL[4]\AR
\NC 0 \NC 0.00 \NC 0.000 \NC 12.50\NC 13.75\NC 15.00\NC 12.25\NC A\ \NC 
27.20\NC 24.73\NC 22.67\NC 27.76\AR
\NC 1 \NC 0.08 \NC 0.115 \NC 13.94\NC 15.34\NC 16.73\NC 13.66\NC A\#\NC 
24.38\NC 22.17\NC 20.32\NC 24.88\AR
\NC 2 \NC 0.17 \NC 0.222 \NC 15.28\NC 16.81\NC    18.34\NC    14.97\NC 
B\ \NC 22.25\NC 20.23\NC 18.54\NC 22.71\AR
\stoptable


diff:
\NC\DC\DC\DC\DL[4]\DL[4]\AR
\NC\DC\DC\DC\DL[4]\DL[3]\AR
\NC\DC\DC\DL[4]\DL[4]\AR


There is (probably) a correct way to solve this.

.Floris