: Model formula question

Hi,

I have a data set with a continuous predictor X, a factor A and a continuous 
dependent
variable Y.
I am trying to build a linear model of the form:

Y = (b0 + b1*X1)*B(A)

where B(A) is a constant for each level of the factor A.
I am not quite sure how to formulate the appropriate model formula. If I 
write:

Y ~ ( 1 + X)/A

, I get estimates for as many constants and slopes as the number of levels 
of A.
What I really need is an overall multiplicative constant which depends on 
the factor A.

Thanks in advance,

Maneesh