[R] standard errors in summary.lm

[Christoph Scherber]

Dear R users,

If I have an aov object, how are the standard errors of the estimates in 
summary.lm calculated?

Using treatment contrasts, I would like to use the estimated differences 
in mean values ("intercepts") to calculate the mean values per factor 
level, and for these mean values I´f like to use the model output to 
calculate the standard errors.

Many thanks for your help!
Regards,
Christoph.