R help - Aug 2010 - "glmulti": excluding intractions between variables

Hello,

I'm using the "glmulti" package to run models of all the possible
combinations of my variables. However, I am only interested in a few
interactions between them.

I have tried the equivalent of:

1)     mod1<-lm(y~a+b+c+a:b)
        glmulti(mod1, level=1)

2)    mod2<-lm(y~a+b+c+a:b)
       glmulti(mod2, level=2)

3)   glmulti("y", c("a", "b", "c"),
exclude=c("a:c", "b:c" ), data=mydata)
where "mydata" is an object reading a text file.

However, the first code only gives me combinations of main effects (without
interactions), the second code doesn't allow me to exclude the a:c and b:c
interactions, and the third code doesn't allow me to define my variables as
factors becuase it uses character strings (and therefore runs a lot more models
than necessary).

Is there a way to use the third code, but tell R that some of the character
strings are factors?

Otherwise, since there are a lot more incteractions that I do not wish to test
than interactions that I do wish to test, is there an alternative by which
instead of having to exclude the interactions that I do not wish to test, I can
simply run models with only the interaction terms that I include in my lm
formula?

Thank you in advance for your time,

Agnese