How to set CTRL and minus as a key mapping?

Hi,
I really like to use big fonts in my gVim 7.4 on Windows 7. But time to time I would like to have smaller font size, to actually see bigger picture (without need to move up or down).

In this case I would like to create key mapping CTRL and minus to shrink font and CTRL and plus to set it back to normal. Something very similar is used in a lot of programs like in Firefox (to zoom up/down).

What I have figure it out I can set the following:
nnoremap <C-x> <Esc>:set guifont=Consolas:h9:cDEFAULT<CR>
nnoremap <C-y> <Esc>:set guifont=Consolas:h15:cDEFAULT<CR>

and works fine, but instead of C-x and C-y I would like to set C-- and C-+
like:
nnoremap <C--> <Esc>:set guifont=Consolas:h9:cDEFAULT<CR>
nnoremap <C-+> <Esc>:set guifont=Consolas:h15:cDEFAULT<CR>

But when pressing CTRL and minus nothing happens, the same with CTRL and plus.
It looks like there is something else going on. How to debug this problem? How to map "CTRL and minus" and "CTRL and plus" to above setting?

Thanks

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