> I'm running 3.09 MinGW.  When trying to compile like this...
>
> ocamlopt.opt -warn-error A -I . -o A.opt unix.cmxa str.cmxa nums.cmxa
> libB.cmxa A.cmx
>
> ...I get this error...
>
> gcc: @F:\DOCUME~1\user\LOCALS~1\Temp\camlresp33a735: Invalid argument
> Error during linking
>
> When I fiddle around with it, I discover that the problems come when I
> add another object to be linked.  This can be either my own binary or
> one of the Caml included libraries.
>
> Can anyone explain why this might be

You're using a set of MinGW compilers that do not understand the
@responsefile notation for bypassing Window's stupidly low limits on
command-line length.

> , and what I can do to remedy it?

See the installation instructions (file README.win32), paragraph
starting with "Do *not* install the Mingw/MSYS development tools from
www.mingw.org".

- Xavier Leroy